One of the most unreliable fields on a Steam's store page is the release date field. Although there is some real actual day in history upon which each and every game on Steam was truly first available to the public for download, the "release date" field doesn't fully and accurately capture that value. Instead, the developer can stick just about anything they want in there, and they can change it even after the game has released. You can't fault developers too much, however, because the release date doesn't really make it clear what it's supposed to mean.
This is a companion discussion topic for the original entry at https://www.fortressofdoors.com/we-dont-actually-know-when-many-steam-games-were-released/