This is a joke, probably it's a sucker bet because Trump had no chance of changing the votes, therefore he will loss pending the formal announcement of the election, I doubt there are still bettors would would take Trump despite the news all over that Biden is 99% likely to win the election.
It's more like >99.9999%. Trump has no realistic chance of overturning the results. His only chance would come if by some miracle it can be established that the Democrats managed to cast millions of fake ballots. However, the gambling sites are trying to make some profits, by allowing such bets. Hardcore Trump supports may make these bets, and lose money in the end.
I don't think the result will change after the official count and besides trump has already lost his supporters and allies. Now that everything has been cleared, he need to accept his defeat in a professional manner and step out of the white house like a decent man.