What's interesting is he never actually signed a message with this key, as far as we know.
Signing a message is not mandatory. If Satoshi Nakamoto already announced his PSG key and his Bitcoin addresses, when he comes back, he can sign messages from those keys as part of evidence. Maybe we will need more than just signed messages to verify Satoshi Nakamoto return.
But signing the same message from both bitcoin address and PGP key would certainly be a good start for proving that he was back.
Similarly for proving ownership of satoshi account in Bitcointalk.
His account is locked, so no. If he wants to claim his account, he'll have to contact me with a PGP signature.