This is bullshit, US presidents have been signing laws etc by autopen for decades.
The times I've heard about it has been the president traveling, but in the age of printers and scanners, the autopen seems strictly worse. Coming home with the original wet-ink signature a week after sending a digital copy seems preferable to the "just trust me bro" method of delegating a signature there is no legal will to legislate a real delegation process for.
Obviously, you can ask Biden: did he authorize such and such a document or not.
This suspicion would only be a problem in the case of someone deceased.
People have been forging signatures for centuries.
The whole thing from Trump is bullshit. “Plausible justification” for those that want to believe.