In the room where the lever is with the revolving wall-door the horde could not reach meg properly.
It sounded like they were pooled up behind the door and on top of the room where they spawn i guess. In the vid it’s visible that they clip through the ceiling of the room.
They were slowly dropping down over the course of a couple of minutes although pyro’s ult found its way up there and killed most of them.
!POSSIBLY LOUD AUDIO WARNING!
I like to explore new maps without much bother so this was in recruit if that helps.