Posted October 01, 2016

I haven't personally tested that with every id game as I don't own them all so what you suggest may be true. If I did own them or wanted to know for certain for a given game though I would investigate the "List of DRM-free games on Steam" wiki, as well as the GOG thread here covering how to run various Steam games offline and/or without the Steam client before drawing any solid conclusions myself. In some cases, games may require using some open source or proprietary 3rd party tools perhaps, which one may or may not consider acceptable in terms of qualifying to call them DRM-free per se. That judgment is ultimately up to the individual of course in lieu of any global industry standardized definition of what that term actually means. :)
Having said that, I just did a quick look on the List of DRM-free games on Steam wiki and Quake, Quake II, and Quake III Arena (all available on GOG) are all listed on the Steam DRM-free wiki using the recommended solutions provided there. The individual gamer out there would need to decide for themselves whether they consider those solutions to meet their own definition or not. I'm comfortable with the recommendations provided there personally, and would in fact likely want to take it a step further by using one or more of the billion other forks of id's source code to play the games. Purists that want to play the original games in their closest to original form as possible using no 3rd party solutions may rightfully feel differently about this as would be their right to do so however.
So IMHO, my claim isn't inherently untrue, but rather it is in the eye of the beholder. If we look at Quake IV, Rage, the newer Wolfenstein games and the new DOOM game however then yeah, sadly there are no viable ways for any of those to be played in a way one could reasonably consider to be DRM-free or even reasonable-restriction-free. All the DOOM3/Quake3 era games and older appear to have simple solutions to play them without needing Steam however once they have been downloaded/installed. That works for me, although not remotely as nice as a snazzy GOG Innosetup solution. :)