With the version from the Humble Store, you can play it without Steam. That's what I meant with “DRM-free,“ i.e. there is not any copy-protection or digital rights management (DRM) involved.
The benefit that Steam offers to the user is the automatic updating: if there will be any patches, fixes, or other updates to your games/software, then Steam applies them automatically, and there is some convenience functionality when you use a game/software on more than one computer. But again, the version from the Humble Store can be used without Steam.
Unfortunately, it appears that the sale is already over; however, the price is still less than the one on Steam.