My naive understanding (correct me if I'm wrong) is that the Constitution is already the supreme law and nothing can be above it, nor Congress Resolutions. If anyone acts against the Constitution, it must be already illegal. On the other hand, if someone is able to break the higher law and get away with it, he could also safely ignore common law.
In the fact of repeated threats by the incumbent President to violate the law and not peacefully transfer power, the mere fact that the law says that someone is supposed to do something is besides the point. Judges simply have pieces of paper that the appropriate officials almost always listen to. But once the top official in the government signals he's going to disregard the law and by association the Courts, or implies that his three recent U.S. Supreme Court appointees will back him up if a dispute reaches the courts, out of loyalty to him and partisan sentiment, reliance on the government to obey court orders isn't enough.
One of the ways to get a politician who could conceivably have the power to execute an extra-legal self-coup which is what the resolution is aimed at avoiding, is to get the overwhelming majority of the President's natural allies to commit to not supporting his extra-legal efforts before he starts engaging in them as he has threatened to do.
This was successful. Trump's extra-legal rhetoric was publicly rebuffed and rejected by all but about 5 members of his own caucus in the U.S. House out of 185.
Similarly, someone (probably senior civil servants who aren't politically appointed and senior military officers) has managed to get Trump's own political appointees in the Justice Department and the Department of Defense to publicly disavow his rhetoric about not making a peaceful transition of power, sending a message to rank and file soldiers in the military, and to rank and file career lawyers in the Justice Department, that on this critical issue, the President will not be supported by the people who in ordinary times report to him, if he tries to overstay his term of office.
The same individuals, had they not committed to a position pre-dispute and if they had not shared the information with each other authoritatively that no one supports this particular ploy of the President, might feel much more intense partisan pressure and fear to let the incumbent President disobey the law with impunity if first forced to take a position once the incumbent President's self-coup was already in the works.
A collective strategy of securing public pre-dispute commitments to not support the actions that the incumbent President has threatened to take gives everyone whose loyalties may be tested when a transition dispute arises confidence about which side will prevail in that dispute, because the worst outcome for any senior official or politician in a constitutional crisis is to appear to be a traitor to the political winner of the dispute. If it is manifestly clear to the incumbent President that he has no support from members of Congress from his own party, the military, or the rank and file in the Justice Department, for his plan, his incentive to never carry out the threat is much stronger.