he can sue you because it's his intellectual property... no one really works for free and no one will do it for free,
the guy definitely never worked for free...and intellectual property is a very squishy thing here!
First of - incest based games are illegal (like comics) in most countries and the guy will not start a, very likely, international arbitration as he will have to reveal his identity, not before finding the identity of the one committing the alleged infringement.
Second - I have really serious doubts the guy ever paid for any of the models and prompt he used in the game, so revealing his identity most likely he will attract unwanted attentions from real companies that don't want to be linked to a title such as this one, plus they want their money and have the means to get it being legit companies.
Third - Using Unity he is already in infringing Unity3D Terms of Use of their software for several reason, one of which is that you have to start to pay them if you start to make over 50 or 100k (which he does abundantly)
Fourth...You have to prove there has been a infringment and from what I can see I doubt he can prove or is willing to prove anything. It takes time, specialized lawyers, money and, most of all, notoriety...and if I am in China or any other country where they don't give a shit about copyright you are just wasting your money/time regardless of what you put on the table
I can already picture him going to the police to denounce it and the police looking at him/them...
There is no copyright, no trademarks registered in any country, no identity of the developer...for what it takes, I can come up and say I have done it.