This is the first published itch.io game, confirmed by the creator of itch in the game comments. 
https://leafo.itch.io/x-moon

Also, here's the oldest wayback machine snapshot of itch.io that includes a list of games. There were only 183 games at this point! From what I can tell, the oldest games are at the bottom. This snapshot is from the 18th of October, 2013.
https://web.archive.org/web/20131018044302/itch.io/browse