Cannon rejected Trump's motion to dismiss on the grounds that the Presidential Records Act (PRA) entitles him to declare anything his personal records... like the locations and workings of our entire nuclear arsenal, for instance. Had she ruled in his favor, Smith would have appealed that immediately.
But Cannon denied the motion "without prejudice", meaning Trump can raise it at trial. At that point she could rule in his favor and dismiss the case. That would not be subject to appeal. It would be an unmitigated disaster.