No version of Roddick stands any chance against a well-playing Djokovic at the AO or Nadal at the USO. OK, at Wimbledon he could be dangerous but no one knows who would win hypothetical matchups because time machines do not exist.
Let us remember that Roddick won his one and only US Open title defeating the clay court specialist Juan Carlos Ferrero in the final.
Roddick's game was good, but not good enough to defeat a 2019 Djokovic or 2019 Nadal outside grass. Outside grass, once he loses his serve, set over for him. As the US Open is the second lowest Slam, he would stand no chance against Nadal, who is an excellent returner on slow hard courts (top 12 ATP return rating on hard courts). Nadal with 4 US Open is 4 times better than Roddick at the USO. And the level Djokovic played at the AO leave Roddick no chance.
To say "Roddick would have won Slams outside grass in 2019" is so speculative as saying "Carlos Moyá would have defeated Federer at the AO 2018". But of course, Federer wins all hypothetical scenarios and Nadal loses all hypothetical scenarios in TTW, so "of course" Federer would demolished Moyá at the AO 2018, while Roddick would have defeated Nadal at the USO 2019.