I've been going back and forth with Apple about it for a year. We don't feel comfortable releasing the exploit details even though they're being slow. We think enough people rely on Hide My Email for personal safety that it would be irresponsible.
Do you believe the mitigation would be difficult to engineer? If, say somebody else, publicly disclosed the unmasking technique how long would you guess it would take Apple to implement a verifiable fix?
I am guessing you haven't tried that excuse on the users your witholding is leaving exposed.