That message signing protocol wasn't invented back then so it couldn't have been signed then.

Many old keys have been sold or leaked, particularly once they've become worthless.  There have been numerous threads on BCT offering to buy old keys.  That address was swept in 2011, somewhat after signmessage was created.

I wouldn't make a substantial bet on the author of the signmessage being the miner of the block.

Invoking "satoshi" is just clickbait, shame on you.