Rather than signing dates, they should sign a hash of the block header that was mined 10 minutes ago.
That's indeed smarter than a date, but note that I asked to provide the topic ID too.
This address, while unused, could have been created at any time.
The money were sent to a public key, but that doesn't make the address unused, because they were sent to a public key that returns this address. The hash of that public key (which received 50 BTC in 2009) returns this address, so he can't have faked it.
However, that doesn't prove he mined the coins. It only proves possession of a private key. A miner could start selling old private keys, and therefore share his ownership with others.