I don't think it's even a requirement. Games can utilize steam apis to have a better integration with friend list / invites / drm purchasing...etc. But they are not required to. You can just upload a standalone executable without calling steam api at all, steam won't prevent you from doing so. Actually, most visual novel games is in this category. Copy the game file and run it at some other place will just work.

Another example is original sound tracks sell as game dlcs. They are simply downloaded into game directory, nothing is protected.

Steam dont require games to have any DRM. Developer might avoid Steamworks altogether and can also make it degrade gracefully: for instance disable online features if Steam API is not available.

This is entirely up to developer to decide how they use Steam.

PS: I am game developer.

can confirm, I was trying to set up my(is it really mine?) copy of obra dinn for my mother to play. and was unable to get it to run given the time I was willing to invest in it.

I was vaguely surprised because I figured that obra dinn being a small single player indie title would not have any drm. and I don't think it does. I think just linking it to the steam dll(for steam integration) makes the check occur. I suspect the solution is a bogus steam dll. but did not find one in the couple of minutes I was looking.

Note that you can copy a game to a friends computer then authorize them to play it, so good for valve, but I did not want to set my mother up with a steam account. so did not use this feature.