This author did not do their research. They are likely quoting from a fake letter that is not from the actual attacker. I did not personally attempt to verify the signature, but others did and they report it being invalid.
An invalid digital signature is a pretty strong indication that it is the fake. I'd consider this poor journalism as it gives this letter credibility that it doesn't deserve.
My statement was merely to try and point out that one of his quoted sources is likely a fabrication.
> The hacker! Or, at least, someone claiming to be "The Attacker," who wrote this:
which makes it clear enough that he knows what he's talking about.