Perhaps we saw it in TAS? The episode calls this room the Engine Core.i.e. Was there an unseen main engineering in TOS that we just never saw?

That's what they've called it but it's just the same engine room set which went by a variety of names in the TV show. It could be Impulse Engineering, it could be the equivalent of the TMP Engine Room (the one with the big vertical power conduit), it was never clarified onscreen.Apparently "Neutral Zone Studios" has a TOS Main Engineering set.
They moved on 11th November 2024, according to another of their posts.