I am hearing this circular argument all over the place

The thesis is that Trump will look like he has a landslide on election night via post-election polling but the mail in ballots will tilt back over to Biden and this could create a political/information vacuum

Surely the American election system accounts for this and you have a deadline of mail-in ballots to arrive and at X time on X days after the election the winner is declared

My initial thought process is that the Dems are flooding this propaganda as information warfare to prepare to essentially coup the government and throw Trump out even if he wins (which I think will be likely)