Like a lot of BBC Magazine articles, this one is only of the mildest interest and coherence, but I found a certain aspect of it intriguing because it's indicative of a tendency I've sometimes noticed for people to attribute works of highly imaginative fantasy and SF to the author's use of narcotics. Usually, as in the case of Lewis Carroll, this is based on no evidence other than, "Well, he must have been on drugs, because that book is really weird!"
I think that sort of assumption says more about the person making it than it does the author. If you lack a good imagination of your own, I suppose it is natural to assume that everybody is that way, and could only possibly create amazing works of imagination under the influence. It doesn't seem to occur to them that, yes, some people just are that creative.
Not that I'm against trying to be the next Philip K Dick if that's your thing. Just don't diminish the creative power of the human brain by pretending everything weird is down to artificial stimulation.