Technically, the rules do say he can't vote if his game's in the poll. I think it's fairly unprecedented for a dev to vote against their own game though! Kudos. Impressed to see so many paid games in the poll this week, too. It's nice that it hasn't just come down to the game that the most people were able to play.
Unless there are objections I'll mend the rules to say developers can vote for a game so long as they aren't affiliated with that game. It's a nice thought to see developers vote for competing games.