Steam version is highly integrated with Steamworks to the point it'll outright crash if it loses connection with Steam. Whole multiplayer aspect of the game is build on Steamworks. I highly doubt they'll go thru all the trouble decoupling the game from Steam and rebuilding the Steamworks dependent parts just to offer the game on another store. It's far more likely that if they ever consider starting selling the game on Itch or other stores, it would be them just selling Steam keys for the game.
In any case, the developers current plan is to complete the game on Steam first before considering future prospects (whether or not to port the game to Mac or Linux or porting the game to consoles or other platforms) or before committing to anything else.