I mean, if the legislature declares martial law to be over, and the military recognizes that, then Yoon can say whatever he wants, martial law is still de facto over.

If the president says "martial law", and the military says "no", that's not martial law. That's the president saying some words.

It may still be a coup d'etat attempt by the president, but it's not martial law.

So if the military recognizes the end of martial law, the martial law is over.

If Yoon recognizes it, and the military doesn't, that's a whole nother can of worms. Then it's really a coup.