I also don't use Steam, bought the game here, and use the download available here. Game plays fine and is kept updated.
Steam is very bad about verifying game updates, which allows them to publish faster than other platforms sometimes. By which I mean that 90% of updates to games never get checked, just pushed live without any steps to ensure they meet the standards Valve require when you're uploading a game. Inserting malware is hard on a game's launch, but stupidly easy once the game is live. So seeing a game getting updated on Steam first is pretty normal. It's also a bigger platform, so a lot of devs prioritise it over other storefronts.
Hopefully, this doesn't take too long to get updated, though.