Define "stripped down". I honestly don't know how the game makes use of Steamworks matchmaking and leaderboards. Goes without saying that the DRM-free version won't have achievements.
Apologies for the late response but I was actually testing the game for quite a while and it is DRM Free even on Steam i.e. can be run without the client including the very latest updated build. But, just to be sure I downloaded the Humble DRM Free build as well a couple of hours ago (which as samlii mentioned was last updated on 2015-10-20) and it doesn't seem to be a stripped down version at all which I initially assumed so even online is available most likely through Direct IP if I'm not mistaken.
There are ofcourse no achievements as you mentioned, apart from that the one small annoying drawback so far is everyone's nick is "Brononymous" online and the name can't be changed in options because the game only uses the Steam ID (when running with it). The other negligible issue is that even in the DRM Free version it shows a "Steam is not running"
message at the bottom left so folks here will probably rage if it releases with that intact on GOG. I don't think its going to take a lot for the devs to fix as apart from that the game is great and I don't see any reason why it can't be released here on GOG because the Humble DRM Free build has all the options including "Level Editor" as well, though I haven't tested if it can be shared online or not.