constitution. Overturning a state law as unconstitutional per the US Constitution is at the core of hundreds and hundreds of Supreme Court decisions, such as, say, Brown vs. Board of Education. The Supreme Court ruled 7-2 that Florida law and the Florida Supreme Court violated the equal protection clause of the US Constitution. That includes Breyer and Souter. This pertained to the fact that different counties were allowed to use different standards for counting votes, and that the recount ordered by Florida would not have recounted all votes equally.

Then they used valid Florida law to determine that a constitutionally valid recount could not take place by the deadline in national statute.

So no. The Supreme Court did not rule that the Florida court ruled erroneously on Florida legislation. They ruled that the Florida court and Florida legislation produced unconstitutional results. And Gore, by the way, did not get a recount of the whole state, but did get recounts in most if not all of the counties where he wanted one. They didn't change the outcome.