Yeah I'd agree a 3 years inactive, slow as molasses Dempsey wouldn't fair so well here, but we are talking about prime v prime. Dempsey wins by KO mid rounds, Corbett had never experienced someone with his speed, footwork or defense
Before the opening bell, Classic's posters will find themselves shocked when they see Corbett and his giant haircut looking significantly bigger than Dempsey in the ring. That said, Dempsey grew up in the House That Jim Built. Corbett has a few tricks Dempsey probably hasn't seen, and might be better on the inside than one might expect...but Dempsey dominated the larger, wealthier, more global sport that Corbett bequeathed to the world. You needed more than a couple fights on your resume to get to that level by 1919. Also, Dempsey is probably better with gloves than Corbett, since he learned the tricks that multiple generations of frantically-adapting boxers developed after Corbett Dempsey by KO (demographic / era superiority.)