Given the elusive and eloquent nature in which we were provided information, it's hard to tell what is meant by compromised PGP keys. There could have been a larger than known data leak or something of that nature.
Why else would it be stated that none of the standard "proof of persona" we normally use are to be considered acceptable.
This is a mystery indeed. I could speculate, but it doesn't seem right after Lauda left already.
Apart from signed messages, there's always Lauda's distinct writing style 