A fellow who used to post here several years ago, had trail cam pics of a cougar eating on a road killed deer that he placed in a spot the cat frequented. If I remember correctly, most photos were at night though. After placing road killed deer at known cougar crossings, he found that cougars would often eat on the road killed carcass. He believed the fresh meat only theory until he did his own investigation.
I've personally talked to people who caught cougars in the headlights feeding on roadkilled deer next to the road. I've followed tracks that led back to their old kills that were no longer fresh, but the cat ate what it could.
While cougars may not scavenge like coyotes and bears, to say they will not eat anything but fresh meat is probably not the case.