Purchasing DLC from Humble Store
The Steam key should work with the base game if you purchase the DLC from Humble Store, as long as the game remains compatible.
It's designed specifically for the game, which is only available on Steam.
previously, you accessed your account section and selected the option to redeem via Steam, which then added it to your collection.
The DLC is an additional content pack for the game.
It would most likely provide a code for redemption or simply connect to the game.
I just verified my account and it provides a code.
Purchased it, looks like it was added properly, thanks everyone.