has fantasy author Terry Pratchett really said more rational quotable things than Einstein, Darwin, Descartes, Dennett, Jaynes, Aristotle and Sagan?

Yes (emphasis mine). As an author, especially an author of humorous novels, Terry Pratchett has much stronger pressures to generate text which humans find aesthetically pleasing than famous scientists, who can count on prestige to carry their words to the world.

I don't know ... there are plenty of references to people in comments in the style of celebrity fawning, just that they are niche celebrities instead of mainstream ones.

Every single one of the names you mention are community celebrities, and yes, we fawn on our chosen ones. However, when I say that we should include more celebrity references, I don't mean we should fawn on celebrities more (neither the community's or mainstream), I mean that we should be more willing to talk about contemporary gossip, give examples about very widely known people, etc. I know it sounds strange to suggest the former, but I think that having topics which we can use when attempting to interact with normal folk on a non-intellectual level can only help us evangelize, break down stereotypes around rationality, and actually interact with normal people, which I'm told can be intrinsically rewarding. The latter just helps reduce pointless inferential gaps created by culture, not knowledge (for similar reasons, we should avoid the accumulation of jargon).