This case can be complicated and in the past, there was discusion on account sellers are ready to sell their Bitcoin wallet / private key so that the account buyers can sign Bitcoin messages too.

I don't say the person who uses this account is not a real owner, but a signed message can be not an enough proof in this case by some coincident activities at the same time on this account.

It can not be a hacked account, but can be a sold account and the new owner has access to that wallet and private keys.