Copyright.
Copyright has nothing to do with intent, or with whether or not you intend to sell it or otherwise make money off it. It literally refers to who has the legal right to make and distribute copies. And even if it’s something that is done by others, it’s still copyright infringement. Publishers who pursue copyright infringement case don’t really care if you’ve made money on it or not. Well... they care if you made money off it, and will seek even more damages, but the point is that copyright lawsuits have been filed against people for simply using it in their YT videos.
Similar example: people put up their own “lyric videos” of their favorite songs and boast quite clearly in the YT video description, “I do not have rights to the music. All copyrights go to the artist and copyright holders.” Well, that’s basically the same as plastering across your video, “I’ve stolen this intellectual property that is clearly owned by someone else.”
Using a game engine for a short film that you claim as your own intellectual property struts right through the middle of copyrights. At the time, you described your short film as a unique visual style, so to watch and see that it’s just game engine footage pretty much discredited the advertising strategy. Not even crediting the source is just twisting the knife after it’s already in. The authors of “Rome: Total War” may look the other way or not bother to go after this stuff, but that still doesn’t make it right.
My point is that it’s always questionable when a content “creator” uses someone else’s intellectual property in their own production without permission or licensing. That’s the kind of practice that can create a reputation that will follow you throughout your career. That’s not a reputation that gets you very far.