Do you have a source for that?
I've never litigated a waiver of attorney client privilege, but, if X emails a friend and says "I've been meeting with my lawyer about that hobo I killed last Christmas. He is very concerned that I'm going to be convicted." Then X can't use attorney-client privilege to keep that email out of court, but it does not follow that the DA can call X's lawyer as a witness when X is tried for killing the hobo, much less that the DA can call X's attorney in front of the Grand Jury, to testify about all of the other hobos X told his lawyer that he has killed.
Possibly the subtle difference is that the letter describes, not merely the lawyer’s concern over conviction, but direct culpability over a specific element of the cause of action, knowledge that the claims of the book were false, knowledge held by both RH and McGinniss. In your hobo hypothetical, it would be like saying you told the lawyer you did kill the hobo, and showed the lawyer the weapon. Loss of privilege would allow the prosecution to explore anything reasonably related to the exposed subject matter, would it not?