No you can't be sure, because there could be the scenario where someone finds that private key. Let me explain, there are a lot of users right now doing brutforce to the bitcoin private keys, someones do it to solve "the bitcoin puzzle", some others attack targeting the address with balance, and some others are just trying to fin a collision (2 different private keys for the same address, so, if any of them find by mistake the PK of the address on the genesis transaction, that doesn't mean he is satoshi.