It's a fan game you absolute buffoon
Maybe perhaps, i don't know, simply ask? Politely? I mean, i hate people who steal other people jobs but if the developer is using content from another person, either he know that developer, he asked for permission or he stole it, and oh surprise you obviously have no way to know which one and simply assumed it.
Having no way of knowing it was sanctioned also means having no way of knowing it wasn't. Which also isn't true, because, you know, you could've used this amazing secret technique known as "asking." It's a far better approach than being a cop.
I know you're trying to apologize, but it doesn't seem like you understand that jumping to accusations was the problem, not you simply being factually wrong.
And that's forgetting the fact that it's very obviously a fan game.
When I posted the link i figured he would say why is that guy using my stuff or say something like it's okay I gave him permission. Something to that effect. It was my way of asking. But now thinking back. I should been a bit more straight forward. That was my mistake. Well what's done is done. All I can do is say sorry and that I'll be more careful next time something like this happens. That's all I can do.