The funny thing about this quote is that it enunciates precisely the understanding that STO has and works towards: all is one, all others are Self, there is no separation. In contrast, STS pretends that is not the case, thinks that they can benefit self at expense of another. At one point, as both Ra and anagogy point out, that pretense stops being tenable because you cannot merge your (individuated) self into the Whole unless you are willing to recognize that the separation was illusory to begin with. Therefore STS has to grudgingly quit its entire game in order to embrace the perfect oneness that is ever the true state of things under the surface of illusion.
STO is defined by embracing the All, STS by intentionally using the other for personal gain. Again, STS is logically incompatible with oneness. Clinging to the pretense that there are other-selves to be exploited is inherently dualistic; as long as any hint of that remains, oneness cannot be realized by an entity. There is simply no matching barrier for STO which already embraces oneness in increasing degrees as it progresses through the densities.
Perhaps the confusion with the view I've seen expressed here can be clarified by recognizing that there are two levels of being to consider: the Eternal Truth of oneness without separation on the one hand, and the conceptual, ultimately thought-form stages of evolution which we call densities on the other hand. Separation is a concept within the mind of the One Inseparable Being; absolute harmony - ultimate harmony beyond harmony, which is oneness - is its nature. These two levels are very different from each other, and recognizing this will explain why STS exists in illusion only, and therefore must ultimately fall away; while STO describes the True State of Being, reflected within the illusion.
Any degree of apparent separation is ultimately a pretense by the Inseparable One; we as the players are placed in a position of pretending that it's real and playing out our roles. However, the game is always about returning to the true state of being, or oneness. It is not about accepting the illusory as real. In spiritual evolution, illusions of all kinds are transcended and gradually discarded, and the oneness of the consciousness of being, which is the truth concealed within all illusion, is embraced. STS is a path whose concepts have no correspondence in the reality of the One Consciousness, but are rather toyed with for a while as, shall we say, an intellectual exercise.
As an analogy to STS, consider the physical body - another temporary and illusory artifact of creation. It exists in concept within Infinite Consciousness, and as a concept is toyed with and explored; but there is no physical body in the True Reality of being in which the Infinite Consciousness dwells. Therefore - and here's the key - spiritual evolution does not consist of or require seeing the physical body as being equally real as the Creator-consciousness within. It isn't. Similarly, the STS path is a temporary conceptual exploration of a being whose true state excludes any disharmony whatsoever; and the absolute harmony and love of every bit of the one for the whole is what characterizes the STO path. Within the illusion, the STO embraces and lives according to the truth that exists outside of illusion. Hence it is called the path of That Which Is. It embraces the Oneness and Holiness within the apparent STS, while recognizing that STS has no existence in the reality of the infinite consciousness, but is a concept and a thought created by it to explore, just as physicality is a concept and a thought created by it to explore, from within, shall we say, its comfortable armchair of absolute blissful and inseparable oneness.
I am sorry if this isn't as clear as I was hoping to be able to express it, but, essentially...
TL;DR - what anagogy said.
STO is defined by embracing the All, STS by intentionally using the other for personal gain. Again, STS is logically incompatible with oneness. Clinging to the pretense that there are other-selves to be exploited is inherently dualistic; as long as any hint of that remains, oneness cannot be realized by an entity. There is simply no matching barrier for STO which already embraces oneness in increasing degrees as it progresses through the densities.
Perhaps the confusion with the view I've seen expressed here can be clarified by recognizing that there are two levels of being to consider: the Eternal Truth of oneness without separation on the one hand, and the conceptual, ultimately thought-form stages of evolution which we call densities on the other hand. Separation is a concept within the mind of the One Inseparable Being; absolute harmony - ultimate harmony beyond harmony, which is oneness - is its nature. These two levels are very different from each other, and recognizing this will explain why STS exists in illusion only, and therefore must ultimately fall away; while STO describes the True State of Being, reflected within the illusion.
Any degree of apparent separation is ultimately a pretense by the Inseparable One; we as the players are placed in a position of pretending that it's real and playing out our roles. However, the game is always about returning to the true state of being, or oneness. It is not about accepting the illusory as real. In spiritual evolution, illusions of all kinds are transcended and gradually discarded, and the oneness of the consciousness of being, which is the truth concealed within all illusion, is embraced. STS is a path whose concepts have no correspondence in the reality of the One Consciousness, but are rather toyed with for a while as, shall we say, an intellectual exercise.
As an analogy to STS, consider the physical body - another temporary and illusory artifact of creation. It exists in concept within Infinite Consciousness, and as a concept is toyed with and explored; but there is no physical body in the True Reality of being in which the Infinite Consciousness dwells. Therefore - and here's the key - spiritual evolution does not consist of or require seeing the physical body as being equally real as the Creator-consciousness within. It isn't. Similarly, the STS path is a temporary conceptual exploration of a being whose true state excludes any disharmony whatsoever; and the absolute harmony and love of every bit of the one for the whole is what characterizes the STO path. Within the illusion, the STO embraces and lives according to the truth that exists outside of illusion. Hence it is called the path of That Which Is. It embraces the Oneness and Holiness within the apparent STS, while recognizing that STS has no existence in the reality of the infinite consciousness, but is a concept and a thought created by it to explore, just as physicality is a concept and a thought created by it to explore, from within, shall we say, its comfortable armchair of absolute blissful and inseparable oneness.
I am sorry if this isn't as clear as I was hoping to be able to express it, but, essentially...
TL;DR - what anagogy said.