Seems like Apple is tacitly acknowledging that sophisticated actors have successfully been man-in-the-middling iMessage users. I wonder if they have clear evidence of that since I haven’t seen any coverage on this.
I don’t get that impression. Given that iMessage is such a high value target, I wouldn’t be surprised either way, but adding more security features is not a tacit admission of compromise.
Agreed with the sibling comment. To quote Apple, this feature can "detect sophisticated threats against iMessage servers". Essentially it's to protect against state-sponsored attacks MITMing you. Doing that also probably requires the attackers also have access to some root CA private keys so it's a very small pool.