I'm a bit lost here. What's the scam part? I mean, there's clearly a fully functioning game engine, that's open source, with public development on github, right?
It does sound like there was poor expectation management in terms of bug fixes, release timeline, milestone scope, etc. The author sounds like a domain expert with game engines but not a coder, so it makes sense they would not realize how wildly unreliable estimates can be. The point about a PR not getting merged and times when the maintainer didn't seem to understand core domain concepts does seem a reasonable thing to be concerned about.
I'm not seeing anyone taking money and running.