This sounds reasonable. I understand why people here would want a steam key for buying the game early (as did I), because it feels like we got "punished" for supporting the game early with no expectations of a steam release.
But on the other hand, it's really not that much money and it's basically a solo dev project. He could have left the itch.io version to rot but he's working on implementing the new (possible) features into this version too.
From what I understand: The itch.io version will get updated with the game's new items, but there will be no steam features such as achievements, leaderboard, and multiplayer. I'm no gamedev but it makes sense that these steam features can't be implemented into the itch.io version.
What I'm saying is: just support the dev he makes cool games. Cut him some slack, don't break his balls over a few bucks.