Why do you think that account belongs to him personally if he signed the message from some old address? As far as I remember from some posts of those who sold BTT accounts, they included the full package - which means the original e-mail, and private keys for all publicly published addresses.
First of all, I don't know how many accounts actually bought/sold with all the things you mentioned. I don't remember seeing such threads. Probably these thing happened a long time ago which I don't know. But I came to know that account selling is not prohibited and people used to buy sell account back in the old days. Even it was normal to collateral BitcoinTalk account to take loans.
The 2nd thing is, if that is the case, why do we even ask for a signed message from an old address knowing the current account owner could provide a signed message from an old address? It becomes a standard to ask for a signed message from an address and this is the reason people stake their Bitcoin address. This is why I thought It would be okay if he can sign a message.