To be fair, when you're on Patreon it's not the smartest move to tell strangers online that you're going to break their rules.
I think the smarter approach is to strongly deny everywhere and to everyone that your game is about incest or that it will be part of it and then just release a patch anonymously, you could even fake being mad about the patch too, lol. Or, simply use something else than Patreon and create whatever you want to without having to worry about anything.
There's a certain dev, which I won't name for obvious reasons, that's been around for a long time that does something like that, to the point that some of his Patreon supporters are convinced his games have nothing to do with incest.
Of course, one of the problems with Braindrop is that he never hid the relationship between the characters so patching in incest would be a lot more suspicious and time-consuming because it would need to add actual renders of scenes with those characters. Another thing is that nowadays it seems like he's lost interest in the game and just sporadically updates it so he doesn't lose supporters so he's not going to put his cash cow in jeopardy for very little possible gain.