There never was any evidence they were scavengers and numerous fossil examples that T. Rex did infact hunt large herbivores for food. The most notable being a T. Rex's tooth found in a hadrosaur's spine, and the wound showed signs of healing meaning the hadrosaur was alive when it was bit and survived the encounter.

"We not only have a broken-off tooth embedded in the bone of another animal, but the bone has healed over the wound, and a nasty wound it was too," said David Burnham at Palm Beach Museum of Natural History in Florida. https://www.theguardian.com/science/2013/jul/15/t-rex-tooth-embedded-prey-dinosaur

Other evidence includes fossilized footprints suggesting a group of T. Rex of differing ages stalking their prey. This isn't to say the T. Rex wouldn't take advantage of a dead animal and opportunistically scavenge, as any large predator will today, but this probably wasn't their primary means of getting food.