As he didn't mention it in the post, it's not clear to me that Substack bans writing by sex workers due to terms imposed upon it by Stripe. Why do you think that this is the case?
I was being cynical, but I think it's a safe assumption. Visa/MC guidelines are usually the reason for bans on sex-related content elsewhere. I really doubt they have morals here. They just want to drive engagement/revenue, lower their expenses on moderation.