Out of all the sovereign citizen nonsense I have seen, this is probably the most likely to work. Not in a “that is the way the law works” kind of way, but in the “the other party might actualy get duped” way.
Essentially, it is a variant of the fake invoice scam. In a fake invoice scam, you send a bill to a company you never worked for. Normally, the company will look at it and ask “what is this about?”. However, occasionally the bill will land on top of a pile of paperwork. Then a parent who was up all night with a sick kid will come in in the morning, see an unpaid bill, and write the check before having their morning coffee.
Essentially the same thing happened here. The bank got paperwork from the IRS saying that the bank forgave the loan (point 1 to the scammer for having this come through the IRS). Of course, most of the time, the banks response is going to be “no we didn’t”, at which point the scammer looses. But occasionally an employee at the bank is going to mess up, and do something that might result in the loan actually being forgiven.