The things said before are exactly right: CEG is a completely optional, second-level DRM that Valve offers to Steam developers/publishers, and its implementation is 100% up to the developers/publishers themselves. The standard client requirement-DRM is also optional, as it's been made obvious by the list of DRM-free Steam games on the wiki that keep updated in this very thread.
Both Witcher games (and AVGN Adventures, as mentioned above) do in fact require Steam to run, but neither one has CEG, meaning that they can be copied/pasted with an offline Steam client as outlined in my guide. If you were to take a CEG game and try it, like Spec Ops: The Line, the game might launch offline, but it will crash until you go online and re-download machine-specific files. Luckily, CEG is barely used at all, and even Valve itself has patched CEG out of some of its games, like Portal 2. As far as I can tell, Linux doesn't use CEG at all in any games.
I actually find that quote from CDPR interesting, because they DO have complete control on Steam. Hell, not only with CEG, but they can choose to not even require the basic client DRM if they don't want to, and I'm kind of surprised they haven't done that so far. At least the Steam versions can be redeemed on GOG.com for DRM-free versions.
Thank you (and the others) for the useful insights, added the client requirement in the post I linked above, and now at least I know I wasn't completely in the wrong on it being optional.