Trump no longer has executive privilege because he is no longer the president. The privilege goes with the office, that is clear. So Biden is the only person who can currently decide whether to assert executive privilege or waive it, even regarding possibly privileged stuff from past presidents. In fact, Biden has already waived executive privilege for records from the Trump presidency. That was Biden's call, not Trump's, because Biden is the one who is president.
Of course, that reality cannot stop Trump and his cronies from claiming that Trump can still claim executive privilege. Trump has tried. It just doesn't standup in court, even in the current Supreme Court with its solid conservative majority hasn't stood for it. Because Trump's claim that he has executive privilege is simply bullshit
So why the fuck is the Washington Post pretending that Trump gets to decide whether to waive executive privilege abut Steve Bannon's testimony? Sure, Trump might announce that he is waiving the privilege, but if he doesn't have the privilege in the first place, that is as meaningful as my announcement that I am abdicating the British throne.