Dude, New one on me, too. See the O-ring? Look up 4L60 images. It looks like the Reluctor gear is in the case, and the sensor probe resides in the center once installed.
Looks like there are external wheel speed sensors in the newer GM vehicles...
As far as what the Mechanic sez, IDK, but it looks like to R&R the sensor alone wouldn't be going in deep.
Disclaimer:
A used Tilesetter's probably notchyer best source for Mechanicing information...