What more did Apple say on the phone? No rationale given?
What the author claims is a vulnerability isn't one in any real sense.
They basically "complain" that the mailbox on the iOS phone doesn't "validates" that the server is a real exchange server, in all honestly neither does outlook, nor most other client-server protocols I know off if the server isn't correct it will fail that's it.
Even if there was a way to downgrade or redirect traffic if you have the ability to do that it's game over, if you can MitM an SSL connection between an iOS device and it's server faking a layer 7 handshake isn't really hard to do at that point.
In this case, it's POST'ing the credentials before the certificate is validated.
This is the step that the author gracefully has omitted: https://imgur.com/a/OXIal, and no nothing is sent until you approve the certificate.
I can't be bothered to set up an actual exchange server to check what happens if you change the certificate midway, however having to approve a certificate change before when the certificates were rotated at my previous employer at least twice on an iPhone (and much older than the current iOS) it would bet that nothing is being sent.
What isn't "validated" is the fact that there is a real exchange server beyond the HTTP/HTTPS endpoint.
You can do a layer 7 validation and validate that there is an actual exchange server beyond the HTTP endpoint but as I stated it's utterly useless since if you can MitM the SSL connection answer "yes I'm a real exchange server" won't be a technical problem at that point.
The author basically sees something that isn't there, the mail app tries to connect to an exchange server, it does so over SSL, it does validates the SSL certificate or prompts the user to approve it if the chain cannot be validates, it does however sends your credentials in the request to the server like it should.