I think he could. Only reason I don’t think they’ve done it is because Hunter still has a trial on the books for September (I think on federal tax charges) and they haven’t yet filed anything with the court in his gun case letting them know a sentencing hearing is unnecessary.
Hunter will absolutely be pardoned before he ever sees a day in jail, but to this point I think they were going to wait until the election (and find reasons to delay any sentencing or jail time) so Biden could pretend he’s respecting the rule of law or whatever his bullshit line is.
If they toss Joe to the side, there’s no political reason for him not to pardon Hunter.
That all goes for the rest of the family the DOJ has been protecting from accountability, too.