I think most people would say "yes," that the concession was premature and that the law would recognize the vote count, not the concession. This is backed up by examples of retracted concessions, meaning that a concession is not a permanent decision.
Add to this the Electoral College. If Bush were to concede and Gore still failed to win 270 Electoral Votes (because Florida still voted and certified for Bush), then he does not win just because Bush conceded. He would have failed to win the majority in the Electoral College, and the House of Representatives would choose the President.
Where the 20th amendment would apply is if the winning candidate declines the office (which is not the same thing as conceding the election). If Bush were to decline to serve after winning the election, then Cheney would become acting President until a permanent President is chosen.
The actual vote is the law, not the candidate's words.
-PJ
Nowhere in any of the posts I wrote on this topic did I even allude to the scenario you described. Such a scenario when the actual winning candidate concedes is exceedingly rare and when it occurs I’m sure the juristicional laws and regs would apply. I have said multiple times that concessions are PRECEDENT. If you and others can’t get that, oh well.