Since the deadline for ratification, part of the Amendment, was effectively passed by a 2/3 vote in each house, Congress should have made sure the extension was passed by a 2/3 vote. Since Congress simply treated the extension as a law (presumably a simple majority in each house), that will enable them to resurrect or eliminate the deadline by simple majorities via precedent.
I see your point. But it will be the Justices who will get the final word. Not just on what Congress did/might do, but states that rescinded ratification. Once we will be in the hands of the all-wise SCOTUS.