It's not really related to the topic, but when the website says you can get the game "DRM free and redeem it on Steam", does it mean that you get both a DRM free version and a Steam version, or do they not consider Steam to be DRM?

Both. You get a standalone installer which works like those of GOG, usually (but not for this game) a Linux and Mac version + Steam key. Even the "Steam only" games are often DRM-free, if you can do Linux. And you can get all of it simultaniously. And the devs repeatedly said the biggest cut they get is from the humble store.

And you can download the game via torrents!