It's a sandboxing method, and atmfd was moved into a sandbox to reduce the risk of people exploiting bugs in it. At this point it is clear that sandboxing it was worth the effort.
Fair, that link points out the Kernel hosts some sandboxes directly. I had assumed they mostly had to be opt-in at the application level (opt-out in the case of the UWP platform), given the name, and those Panes are open third-party extension points and it does seem like sandboxes should apply to anything running in them as well, so it seems a fair assumption to believe that is where they were applied.
It's a sandboxing method, and atmfd was moved into a sandbox to reduce the risk of people exploiting bugs in it. At this point it is clear that sandboxing it was worth the effort.