i.e. you can spend them.
You can ONLY sign a message if you have the private key ... which also means you can spend those bitcoins.
The address I listed is the first mined block - which clearly WOULD be 'satoshi'
Do you give other people full access to your Bitcoin wallet?
Of course not.
i.e. satoshi's 'friends' would not be able to sign messages with the first block address.
Though, of course, 'satoshi' is very unlikely to be only one person ...
Edit: aside, CW claimed he had the keys for block #1 and #9 ... and that he did sign a message with them - but it was faked.
So really it only required him to do what he claimed he could do to prove it.