Bitcoin are associated against addresses which are held in wallets. To transfer coins away from an address (i.e. to spend them or to sell them) you need to create a transaction on the blockchain - as part of doing this you need to “sign” the transaction with a private key associated with the address which holds the bitcoins.

In this case the guy doesn’t have an extra copy of his private key so cannot transfer the coins - he still “owns” them but cannot transact them. It’s like having gold bars locked in a safe but you can’t remember the combination - except the combination is so huge that the chances of guessing it are effectively zero.

Most people who hold more than a trivial amount of bitcoin will have backups of their private key or use mnemonics to remember it but in the early days when 8,000 bitcoins were worth pennies there was no real incentive or knowledge that it was a good idea to keep backups of the key.