If these memos were binding on future administrations, before leaving office, Trump could have had his DOJ create opinions that would bind Biden. Nonsense, of course.
Politico implies that Biden can say "HA HA I'm not complying because of a memo that the Trump administration created."
There are lots of reasons Biden won't (and shouldn't) do that.
If subpoenas are frivolous there are ways of challenging them in court.