Does that mean we get locked out of the editor? It looks like if it isn't on Steam, we won't have access to a bunch of content. I have seen several games like that already. Does anyone have any more information on this? Steam already went full on DRM on mods now.

That depend on how they'll decide to implement it... it could also be "compatible" with Steam Workshop, but also able to work stand alone... the informations are still vague for now...

For example the game "Megaquarium" has all the MODs on Steam Workshop, but those are downloadable from there even without a Steam account, so them can be used in the GOG version of the game without issues at all, and without having the game on Steam too.
In case of Megaquarium anyway, those MODs don't require a game editor to be made, the Steam Workshop is only a container.

In the end they implemented the editor for GOG version in the right way, it works and there is no Steam Workshop integration, but you are free to put in the editor folder everything you want, like the main campaigns taken from the game installation, or even maps downloaded from Steam Workshop (but naturally you'll need to get them from Steam using Steam-related tools not included in the game).