They've communicated this terribly but the intent is supposed to be that they're testing the new embedded comments system by restricting it to just supporters for now. Apparently it's supposed to be rolled out to everyone eventually. And also apparently it won't take the place of the forums for longer form discussion and those that want it. Though I have my doubts because it's obviously positioned to be more accessible and apparent, which will obvious starve the forum chat and potentially kill it.
I'd like to say that the problem here is mostly that the MDex staff did a crap job of doing this in a way that would stop people from drawing the wrong conclusion and getting upset about it, but it also is kind of a dumb setup that was naturally going to cause more problems than it fixed.
For me its the way they implemented it. Clicking on comments on feed/updates or on manga main page chapter list - instead of opening comments - opens that chapter with this new panel thingy. It instantly marks that chapter as read/seen. So if I want to read comments on the forum for now, I need to go to forum and look for manga/chapters threads there, or click on the link in the panel, then go back to main manga page just so that I could remove the seen mark if I want to keep status of what I saw clean.
I hope that when this thing is tested and released and if its as you say - comments on forum will stay - those buttons will revert back to how they worked, and the "embedded" ones would be either separate, or just forum ones just displayed in the manga with that fancy panel. If they somehow change it so that embedded ones will be invoked like they are now while this testsing runs, and forum threads will be separate - requiring me to go to forum and search and stuff? It will probably completely kill any commenting in me. And probably rise the need to find a replacement site strong enough for me to actually consider doing so. A lot of titles already force me to go to other sites cause the only group working was greedy and got banned, or title was DMCAed, so if there was an alternative nice working site, that would let me somehow export MD titles/read per chapter status and just import it into the new site - I would probably be gone from here quite fast.