Kind of looks like the woven sheath of wiring passes into the notch 'U' where you push the clip inwards above the top of the rounded metal fixture. Idea being to keep the fan from rocking upwards and falling out of the bracket
It needed me to spin the assembly then it dropped right in and the clip made sense.
And yeah, the junction box spoons a 2x4 dead center so I couldn't fish the wiring straight up through the mount but it's working now.
Looks tiny compared to the old one, but it kind of is. Still produces adequate airflow though. Now to pull the trigger on a downrod to get the other one in the living room.