Earlier quoted context omitted.
Nobody claims you have to understand category theory to write Haskell. In fact, most will tell you it's not needed. The saying "a monad is just a monoid..." is a cliché and an in-joke, not gatekeeping. It's the community having a laugh at itself.
I've had someone do that on this very website. I was assured that affine types in Rust are too difficult to understand without a solid grounding in Category Theory. The years have proven that ease of programming and the burden of knowledge are the two most important elements of a programming language. FP zealots simply won't accept that their chosen paradigm is opaque to most for benefits that can't seem to be writte…
On this website you will hear anyone say the wildest assertions. There's the whole human range of expression here. But what makes you think actual FP practitioners (and Haskelers) really believe this?
The proof wouldn't be what some random person here on HN tells you. The proof would be you getting involved in an actual FP community trying to write an actual FP project and being told that you just cannot do this unless you understand category theory. Which, as I said, is not something that happens... at least not in my (limited) experience.
Never confuse what people tell you here on HN, random forums or even throwaway StackOverflow comments with what actually happens in the actual communities when trying to achieve real goals and not just chat about stuff.