I believe they're referring to IRL content featuring those actual acts, or deepfake content simulating them with real people.
This is all under this section of the guidelines.
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It's mostly dialing in on real content promoting these acts, rather than a fictional portrayal.
You also pulled the TOS from the wrong Subscribestar website, they have a literal adult variant of the website for pornographic content.
You must be registered to see the links
Differences between the TOS on both websites here
It's also important to notice how things are worded in the TOS.
"We may, but have NO OBLIGATION, to monitor, edit or remove content that we determine in our sole discretion"
Considering how a lot of their top earners on the Adult platform are mostly centered around incest themed games and art, (skuddbutt, breadman, Milda, etc) it's safe to assume that they won't be banning this form of content unless laws and policies require them to. One of their main slogans on the front page is literally this.
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SS openly embraces NSFW content, while Patreon only tolerates it because NSFW creators happen to make up a large portion of their revenue. SS is easily the most trustworthy crowdfunding adult site right now.