First, things in cars do suffer from effects of UV, although the glass does slow this down. My current 21-year-old daily driver has plenty of plastics that are somewhat bleached from the sun, including the once-black carpet on the rear deck. (This, incidentally, is the same manufacturing year as the car that I tested with.)
Second, CD-R media back then was designed to be reactive principally with infrared light (because CDs themselves use IR), of which there was plenty. (I suspect that the dye formulations have shifted with the transition to the shorter red wavelength used by modern optical drives.)
Third, it is plain that IR was plentiful in this environment.
Fourth, it is plain that by your standard, almost zero optical media ever sees significant exposure to direct sunlight.
I don't have a dog in this race. It was simply a curiosity at the time: I'd heard that CD-R media hae a limited lifespan, so I subjected it to the most extreme environment I had at my disposal.
Media included Kodak archival with a gold reflective surface and an extra protective layer over the laquer, TDK Certified+ with a silver reflective surface, and a couple of varieties of store-branded media with the common aluminum surface.
Environment went from crazy-dry and bitter cold, to ridiculous hot and humid, with occasional condensation due to weather changes, and random chemicals and surfactants (from cleaning the window).
I also had a control group of the same data on the same media, stored properly indoors in jewel cases. These, unsurprisingly, also worked fine.
But yeah, I'm sure that it was luck. Or maybe that I made it all up, as if anyone gives a shit about a 650MB optical disk these days.