Which part is the "bottleneck"? Is it the idea that at one point nearly all mammals were nocturnal and that it became advantageous for some to transition to diurnal since there was less competition for resources during the day?
dinosaurs pressured the mammals to stay small, hide in burrows, and roam at night.
dinosaurs, and a lot of other things came to an abrupt end, the basal mammals survived in thier bunkers with hoards of seed, and were relieved of selective pressure, thus diversified in thier traits this is when mammalian diurnalism made a big comeback so they could now compete among fellow mammals radiate and expand niche into the estate of the late dinosaurs
There's lots and lots of adaptations that repair damage to DNA and that deal with damaged cells. So there must have been lots of evolutionary pressure to keep them in place and keep them working.
Animals in the wild probably don't typically die of cancer, because those adaptations are in place.
But you are right, that even mammals active during the daytime have not (re)gained that specific photolyase DNA mechanism that their ancestors lost.