Seems odd that he's warned people about it here but not done anything to have the game removed on Steam cos someone is making money off of his game and it's unfinished on top of that. Very odd.
There are three possibilities: 1. it's his game and he sells it himself, but then why warn people here?
2. there were two people who made the game. They fell out and the project fell apart. One of them put it on Steam, the other had access to the account here and tried to spit in the soup.
3. it's actually his game, but he can't prove anything to Steam because he would have to provide proof of copyright. But he can't because he didn't pay anything for assets, music, etc. The other party who put it on Steam did, however.
Let me put it this way: Steam is very sensitive when it comes to copyright infringements, as they themselves are also partly liable. Should the rightful copyright holder have reported the infringement of their rights to Steam, there is no way that the game would still be available on Steam. However, anyone who makes the claim to Steam will also be asked by Steam to provide proof of this. An affidavit notarized by a lawyer is sufficient for this purpose. Steam would then immediately contact the party that placed the game on the platform and “ask” for proof of legality.
To make a long story short: I don't believe the “someone else is selling my game” story. My bet is on option 2 from my list.