Illusion don't go after people using their assets, they never have and they never will, absolutely no one has anything to be concerned about. If you credit them, which I would never do, it wouldn't matter. They are not interested in perusing the small creative developer who makes vn's.
They only actively go after people who pirate their software, and those who pretend to be third party sellers of Illusion, of which there are no official third parties to them.
I don't know who said that you could not sell HS1 / 2 games on steam, but that is very wrong, because you can. A lot of people get rejected by valve because they have not followed their recommendations, which are not that stringent, but still you get people who try it on and then get butt hurt when they get rejected by them. They are told to make alterations if they want their game to be sold on their store, but for a lot of people that would be too much work for them, so they don't sell their vn, they still tend to make money on patreon or subscribe star so its not the end of the world. It is also not exactly cheap to keep submitting your game to valve either, since it cost $100 USD per submission, and it's touch and go if they accept it or not, seems to largely depend on the people on the day they receive submission, some people there are more receptive than others it would seem. But, yes you can sell HS2 games on steam and on other platforms. Most people don't mention the game engine and renderer because most people know what they are looking at.