And that’s nuts, because, I’ll say it again, it means Trump (and any future president) can take documents clearly marked as Top Secret and containing information about matters like nuclear codes, U.S. battle plans, or information that identifies highly placed human sources putting their lives at risk, declare them to be his personal papers and walk out of the White House with them.

No, it doesn't. Or rather, it doesn't change whether they can do that today.

The law doesn't currently prohibit the President from taking all of those documents and walking out of the White House with them. He's the President. As long as he's the President, he is allowed to take anything he wants from the White House and move it down to his Little White House (whether that's in Mar-a-Lago or Delaware or wherever). All that the criminal law requires is that he give them back when asked. The PRA defense is a defense to the crime of not giving the records back (at least, that's the argument - I think he has an obligation to return those documents whether they're personal records or not). But not really relevant to whether he was allowed to take them in the first place.