what was the circumstantial proof?
The circumstantial proof was that he claimed to have signed a message with a known Satoshi address,
yet upon closer examination, he signed a message with a well known Satoshi signature from 2009.
It is circumstantial because the proof provided was indirect and doesn't prove he is Satoshi.
If the community saw him sign with a new Satoshi signature, it would be proof he controlled that privatekey.
Also, it may be circumstantial as that CSW purportedly verified this signed message in controlled conditions
in which the media (and others) could not independently evaluate that "proof".
The resulting "proof" provided could only have been verified under those controlled "circumstances".
Ultimately, the "proof" was not concrete and unquestionable.