assuming his ult is him jumping and considering that hes jumping at least scaling his ult to his size hes easily jumping at least a good half kilometer at a time. the world records for long jump and high jump is 9 meters and 2 meters respectively so if we assume malphite can do all this. his half km long jump is now roughly a 300-400 foot high jump which is his own height and them some and for a typical backflip u only need to be able to jump half your height if even that. tldr - yes malphite can do a backflip