Lost bitcoins are those which nobody has the access to it. Forgotten wallets, death, losing the keys etc... are all truly lost bitcoins.
"Truly lost bitcoins" are those where there's solid evidence that private key material has been permanently lost. But valid and solid evidence is rarely possible or verifiable. In most cases you have to believe a story someone tells where they claim to have lost access to private keys permanently.
Another case are truly burned bitcoins where there's proof on the blockchain that coins were sent to OP_RETURN or other provably unspendable transaction outputs.
Mt. Gox scam is not technically lost, someone has access to those funds and same with Satoshi's funds which are not technically lost even though we may consider them as lost because Satoshi will never access those funds again.
Unfortunately there's likely no evidence that someone still living has possession of required private key material to spend those coins in either case.
If Satoshi is still alive (which I hope for him), I don't believe, he will ever move any of the coins attributed to him. But this assumption is no axiom. I just see no reason for him to spoil his so far excellent privacy. It's his life insurance...
Sometimes things are not in our control and we often end up losing our money and those will remain inaccessible to everyone forever.
I highly recommend to do some reasonable loss risk assessment and have some mitigation implemented to avoid most relevant loss risks. Self-custody has undeniable responsibilities.