Conceding is not legally binding in any event. Gore conceded for example, then went on to file the FL lawsuits that went to the SC. The election is held, results are certified, electors are chosen, then make their votes, the electoral votes are counted and if you get 270+ you win. If there is no winner the House votes. Nowhere in that process is a concession even mentioned, much less required or binding.
Lets suppose Biden was declared winner of every State election, Trump concedes and the results are certified and electors selected. He's still not President, merely President-elect. Should the electors file 270 votes for Trump, he wins, Biden goes home.
That's actually how the EC was designed to work in the first place. You don't vote for President, you vote for some people to who then vote for President for you. Those people were supposed to be smart and represent the interests of the people who selected them, not mindless automatons that simply carry a voting chit to Washington. It's actually perfectly fine for them to get together and vote for someone who did not even run for office, as long as they are qualified. Some might be facing a fine for failing to cast the designated vote under their State law, but that does not invalidate the vote they make.