There is no auditing whether he sold all his keys in the physical world. They may never have "transacted" on the blockchain, but that does not mean he never just gave away the ability to transact them physically.

He may also have taken out a loan from a traditional financial institution and used them as collateral.

There is a very real possibility that they are in fact owned by someone else now, who has made the same exact calculation you have - they are a valuable asset that has exactly zero liquidity.