It's funny because the community knows some people too deep in the rabbit hole come across that way, but because the community acknowledges this is a hilarious assertion that would alienate newcomers -- if said with a straight face -- then it cannot be gatekeeping.

Gatekeeping would be if the community said this with a straight face and everyone got impatient when you just "don't get it".

> I've seem the claim that you can't really use Haskell without [knowing category theory], here on HN, more than once

I've almost never come across this assertion; it's certainly not common in the community (or wasn't when I was learning Haskell). I can tell you it's certainly false: you can very well write Haskell without knowing or studying category theory.