There is also that, while Biden is President (in ways the other bloke is not), these ...or at least some of them..? date to his time as VP. Which may have given him the rights to possess them at the time, at least part way up to the similar point of "not even should be left on the desk of the Oval Office, without a suitable guard" level, but would not necessarily cover his subsequent time out of Office (and the documents having been taken out
of the 'Whitehouse office').
It would really depend upon what each individual bit of paper is covered by, in rules and regulation terms. Probably some are 'merely' incidental copies that a sensible handling (being actually locked away, having been legitimate take-home summaries of the "real secure documentation", and time-sensitive such that they fulfilled no purpose to anyone much beyond the day they were actually picked off the printer tray) would not justify action, though questions should be asked about others (and will be) that seem to maybe have inadvertently escaped the attentive posession of some serious man with a handcuffed-briefcase from whatever Three Letter Agency the docs normally are locked tight up within. With a sliding scale between.
Perhaps it would be actually
useful to have Biden fall on his sword. If a sitting President can be seen to be unable to go against relatively trivial document handling indiscretions, then it wipes out his predecessor's claims that having
been President means that he can agregiously flout the rules (probably would say that he could stock the common facilities in Mar A Lago with 'loo-paper' bearing nuclear launch codes, should he so wish) by having appropriated so many 'souvenirs in such a blatent way. (And, for example, where exactly are the documents that
were within the empty folder-covers? Handed over to an international 'guest'? Clearly not just forgotten about, in a locked drawer, but at some point conciously handled.)
And I bet the other surviving past-POTUSes are also at least thinking of having a quick check in obscure places, just in case.