He's got the bad version of Alzheimer's.

When my son and I saw him a few years ago, it was pretty clear he was having some trouble. (though he did spend a good 2 or 3 minutes taking to S'Knot about video games, and congratulated S'Knot on finding his picture in "Where's my cow?" which he signed.)

Based on that, and how his writing has changed, AND the fact that I don't know Pratchett himself; I just can't make judgements on whether or not he can comprehend and enjoy a snarky playful mathematics website *now*, but I would be willing to cross my heart that it's the sort of thing he *would* enjoy.