Hardcore Bitcoiner here.

If Q uses Bitcoin to sign a message using the key to an address he controls - we would know 100% it was him.

Example:

The is a guy nakes Craig S Wright who in 2015 tried convincing the Bitcoin community that he was satoshi. The community demanded he prove it by signing multiple messages with the private key know to belong to satoshi.

He signed one transaction, which would be possible to fake under controlled circumstances, but multiple messages under changing circumstances is impossible.

He disappeared for years after the embarrassing thrashing he got from cryptoworld.

Now he is forever known as Faketoshi.

Q could use Bitcoin to prove that he is the one Q, with 100% accuracy