Incoming M1 at the Chicago campus here. Yes, this is correct. They changed how it worked last year so there was some confusion on when to apply, but it's definitely post-acceptance. Basically, at some point (sorry, I don't remember exactly when), the prompts for the concentrations will appear in the AMP portal, and then you can fill them out. Just for reference, I was accepted into IMED, and I submitted my application at the end of March, received my (2) interviews in mid-April, and was accepted near the end of April. And I think they said you can apply to multiple concentrations, but can only be in 1 if you are accepted into multiple.
One other thing to note is that your campus assignment will affect your ability to do a certain concentration. As in, if you apply to a concentration and get assigned to a campus where that concentration isn't offered, then you won't be able to do it, which is why they have it as post-acceptance I think. They also had a couple of Zoom sessions about scholarly concentrations in November last cycle, but I didn't attend so I'm not sure what they covered or anything.
And if anyone has any questions you think I can answer let me know, and I'll try to help if I can. Good luck everyone!
Edit: added which campus I'm attending