OnlyFans isnt a porn site in the same way that Twitter isnt a porn site. Many of the high profile users are in the adult industry but people use onlyfans for other types of content as well.
I'd venture to say that 99% of onlyfans content (that people are paying for anyway) is porn. Just because there's a small subset of non-porn doesn't change the fact that they're selling access to porn. Twitter is a completely different story as they're selling advertising, not access to nudes.