Finally got it up on a lift and looked. Things appear differ from the GM part reference cited in the thread.
On the passenger side, there is a metal bracket anchored to a stud emerging from sheet metal that must have run to the bottom bolt joining the fender liner to the bumper skin. But it's not shown on https://www.gmpartsdirect.com/auto-part ... -rear-scat
and there's none on the driver's side.
Off topic: there's a center fixture unrelated to the bumper that almost looks like it was to hold a spare tire. But no room for one. Wonder why it's there....