Is it possible to securely transfer bitcoin offline?
Like if someone hands you a thumb drive and says “this contains a key for a $1m wallet”, you don’t know if they kept a copy of the key. But is there some mechanism whereby it could be guaranteed that you have the only copy?
No, if the transfer is offline, there can be no guarantees that the same person didn't try to double spend by transferring the same funds to someone else (unless you involve a trusted third party).
No, that's not what the question was. The parent poster specifically said "you don’t know if they kept a copy of the key. But is there some mechanism whereby it could be guaranteed that you have the only copy?" In your example the account key on the usb stick is not guaranteed to be the only copy, and thus, there is risk of the sender double spending that money.
Like if someone hands you a thumb drive and says “this contains a key for a $1m wallet”, you don’t know if they kept a copy of the key. But is there some mechanism whereby it could be guaranteed that you have the only copy?