I mean that in the sense that the devs wanted to expand GMT but couldnt due to the Source engine - so GMT, in a sense, was actually incomplete and therefore technically a beta. Moving GMT to the Unreal engine gave the devs the freedom to break past that Source ceiling, but it doesn't mean we still can't miss GMT.
Not everything that has not yet reached its final state that will not be touched again is beta.
With todays development models close to the user like Early Access, you should probably get used to not seeing the whole thing as incomplete but apply the cycle to single features.
I would consider GMOD Tower to be it's own complete experience.
You are correct in saying the devs moved away from source due to modding limitations but Tower Unite is more of Gmod Tower being rebuilt.