The onus is not on us (or anybody else) to prove that CSW (or anybody else) is not Satoshi. That is not how the burden of proof works. If you make a claim such as "I am Satoshi", then the onus is on you to provide evidence or proof to substantiate that claim. If you fail to do so, then that claim can be completely disregarded, and does not require any further discussion, counter proof, refuting evidence, etc. This is essentially a case of Russell's Teapot. Quod gratis asseritur, gratis negatur.

Having said all that, even if CSW came forward with a signed message from the genesis block (he won't), the mountain of evidence against him being Satoshi is more than enough to allow such a signed message to also be dismissed as a stolen private key or similar.