Terence Mckenna denounces relativism

[Fang]

It is interesting, the level of defensiveness here...

By the that I mean he is discounting data for the sheer reason he finds it 'patently absurd', which is simply his own personal bias

This is the relativism he is talking about "well that's just like your opinion [bias] maaaan".

It is quite ironic this is his position because most of the ideas he supports are patently absurd to most of the scientific community

It would be ironic if it were actually relevant or true. Terence is regarded as a very smart man with theories that are untestable thus outside the reach of scientific scrutiny, what members of the scientific communities personal opinions are on him hardly matters in terms of being "scientific" as the opinions can not be backed by actual science.
This is an address to the once proud and honourable psychedelic community, addressing an issue which was a major cause of it's decline in value to the rest of the world and the decline in honest investigation of the unknown. He is not saying that all ideas should be held under the scrutiny of the scientific method, I'm not even sure why you are bringing the scientific community into this lol

His ideas are "far out" but there is a reasoning behind them, as well as experience, the problem is the experience is far out lol
Any form of reasoning behind an idea is better than just spewing it from your intuition.

So he is actually eroding his own position with this little speech.

He would be if what you said above were true, which the overwhelming majority of people viewing must not think so as they have responded positively to his message.

Guardian, I think he has had a positive and negative influence as well, the negative mainly being due to uneducated fools cherry picking quotes to justify a drug habit or entertaining transcendental notions

These appear to be blatantly contradictory positions from Terence. One the one hand he wants to explore some mysteries of consciousness, and in the next sentence he wants to shut down discussion on others.

If you want to keep moving forward it's important to recognize steps backward.