I believe this screenshot is the closest thing to proof that Hal Finney was Satoshi Nakamoto that there is.
What is not actually provable is that Hal actually made this screenshot, or if the screenshot is not just engineered to make it look like it was Hal.
What is provable is that the hashes in the screenshot do check out, which I verified here:
(hashes from the screenshot):
(corresponding links to https://mempool.space which show they were from Bitcoin's Genesis block 0):
1st hash: 000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f
2nd hash: 4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b