But those are huge and well-established brands; using their names is likely in some cases to cause confusion amongst consumers. Preventing such confusion is, as far as I'm aware, the goal of trademark law.

Moot-the-person on the other hand is quite obscure; nobody knows who he is or associates the word "moot" with him other than a small handful of people in one online community. I.e. it's not a well-established brand, and using the word isn't likely to cause confusion or mis-association for the general public.