Jumbo can look at Kuss' data. Kuss was pretty much his normal peak level at the Tour. Question is can Vingegaard hit a crazy peak in July after being at the Giro in May.If the level is similar to the Giro this year, Vingegaard could definitely win without being 100%. I was not impressed at literally any point outside of the first time trial by Evenepoel and that one very hard dig by Roglic (and Hart) at the hilly stage. Important to remember that Roglic was not at 100% either (I assume) and that Remco got COVID, maybe then it would have looked different. But in the end, that was not only a whack Giro to follow, but I also don't think the level was anything special at all (which probably also contributed to it being such a defensive slugfest).
It's also not just about level, but also fatigue. I don't think the dogshit weather of this Giro would have helped very much.