This is something you'd learn in your first year at law school. If, as Trump's complaint alleges, Dominion did destroy evidence then the evidence is immediately assumed to support the opposing party's allegations.
Biden's defense would now have to prove there was no fraud as if Trump's team had certain evidence of there being such.
Except that in all of the filings I have seen so far, Biden is not a named defendant. So he does not personally have any evidentiary burden in any of these proceedings. Your argument would however apply to the other defendants.
He doesn't literally mean Biden. Biden side.