It's part of the fender liner... 92270537 Fender Liner LEFT 92270538 Fender Liner RIGHT 92138627 Cowl Vent Panel/Fender Liner Retainer
You're correct, they are part of the fender liner. Which is good since I planned on replacing mine due to damaging both front sections (unrelated to this).
It looks like the part numbers you listed replace quite a few parts (92270537 (Left) replaces 92161689, 92251459, 92260897, and 92268803. 92270538 (Right) replaces 92161690, 92250250, and 92260898). Hopefully the updated design fixes this sagging issue.
I am seeing the following note about the bolts and push nuts (emphasis by me):
92268803 LINER,FRT W/H(S/S BY 92270537 - DUE TO LARGER BOLT HOLE LINER/ROCKER MOUT POINT MUST REPLACE WITH BOLTS 92139212 QTY 4 & PUSH NUTS 92139226 QTY 8) - LH
92260898 LINER,FRT W/H(S/S BY 92270538 - DUE TO LARGER BOLT HOLE LINER/ROCKER MOUNT POINT MUST REPLACE WITH BOLTS 92139212 QTY 4 & PUSH NUTS 92139226 QTY 8) - RH
These things are expensive (~$130 list price, ~$90 online). I was thinking about going aftermarket but then started to realize the sacrifice in quality when it comes to a piece like this.
Below are some images from an OEM used G8 fender liner (92161689) from a listing on eBay. The hole used to mount the liner to the underside of the vehicle is intact in my vehicle. It's the section circled in red that began to sag, which eventually was the catalyst to the further damage I created.