Why does the lender have to be a highly regulated bank? Couldn't it be something like a foreign sovereign wealth fund?

I mean, sure. But then....what are we talking about here? I mean, a foreign sovereign wealth fund could give Trump a $200 million mortgage on Mar-A-Lago, also, with a wink and a nod about how there's no need to worry about paying that back during his lifetime. There's no dearth of ways to get money to a President who has lots and lots of active business interests, as long as you're willing to leave a paper trail.

The reason DJT looks different is because you can buy and sell on the open market without anyone knowing you're doing it, so that seems to open up the possibility of shenanigans - but if the SWF is making the loan, rather than buying the securities, then they've created the same paper trail as if they had just loaned him a gazillion dollars against his stake in Trump Org.