Well, there is (d) paranoia. Suppose he does have the private key, but, because it is literally worth millions of dollars, he has it somewhere secure. It's no

Well, there is (d) paranoia. Suppose he does have the private key, but, because it is literally worth millions of dollars, he has it somewhere secure. It's not sitting on his laptop, waiting for someone to steal it (physically or electronically). Signing a message before the announcement was lower risk than signing messages now, after the announcement. Accessing it a second time, with greedy little eyes all over him, is risking millions of dollars to convince people who probably wouldn't be convinced anyway.