You're confusing with Architecte des bâtiments de France. ACMH is a competitive exam and the Wikipedia page [1] can clear that up. I know several people who took it. It's just become very rare, like one every four years. It's definitely not a promotion, and unrelated to the ABF position (which is a part of AUE). But it is equivalent to being a civil servant, except you can have an architecture business of your own.

[1] https://fr.wikipedia.org/wiki/Architecte_en_chef_des_monumen...