Yeah, the ADM certified stuff, as far as I know, is a small percentage of the music on there. The distributors I've worked with don't let you have a separate Apple Music version in the same release (a lot of them don't even let you pick which stores it goes to, it's just bulk submit everywhere), so it's extra work and/or cost to make an AM specific version. And only the top mastering talent seem to be ADM certified- I don't see many advertising it (some do). I figure if an ADM master is made, that one is submitted everywhere, but it's really a minority of the releases. Most of them maybe conform to it by happenstance by way of just making a nice sounding master, but I'm not aware of Apple rejecting loud releases, for example. You can submit a song mastered at +6 LUFS but they will just turn it down when someone plays it.