If there was a Supreme Court vacancy right now and the Republicans were pulling some kind of shenanigan to prevent a new one being seated until after the election, that'd be the greatest gift they could possibly give to the Democrats. That would mean that the election was very obviously going to decide the fate of the Supreme Court, which would bring out tons of voters that might have stayed home.
Whereas if Biden was to seat a new Supreme Court judge right now, and then Trump won, it's not like the Supreme Court would matter anyway.