[nybble41]

> Effectively the investor gets a bigger principal than was actually loaned.

This difference between the amount loaned and the amount paid back over the life of the loan is commonly referred to as "interest".

It really feels like they're just playing semantic games here. Any proper comparison of APRs would include both components. A true "negative interest rate mortgage" would be one where the bank pays you to borrow money, with the total amount borrowed being greater than the total amount paid back to the bank.