I was reviewing a posting from John Hinkley (John Z) from 2015 who stated that GM routinely flash chromed stainless steel trim so I stand corrected. I have a couple of sets of NOS windshield and back glass stainless trim that are not nearly as "brilliant" as the stainless on the Cortez RS/Z (by a mile) which leads me wonder if that trim was triple / show chromed along with everything else that received chrome plating. Can't say for sure but it would appear to be the case.
What you read is in the link I posted. I have NOS and old used stainless moldings in stock. Also highly polished stainless that was not flash chromed for a customer. I've polished stainless myself with all the proper compounds and buffers. The polished stainless screams brightly like show chrome which is unlike the NOS stainless. I doubt that car you saw had the moldings triple chromed, there is no need and a waste.