OF and Fansly isn't just porn. There are many models that use it and show nothing. Many use it for cosplay and hobby, since Patreon would flag their posts due to X, Y, Z and their cut is bigger. It falls under an "independent modeling" category. Using sheer, thin or slightly covered clothes can make them fall under a "I never did nude" as a technicality, in case a company goes after it.
It's like in some nude beaches where even socks are prohibited, only sandals and a towel is allowed going in.
I'm not defending it, but you can really game the system with technicalities in rules and laws.