I don't think this has anything to do with EPL or not. The way you describe it it sounds a little bit as if the EPL was a different sport to European football but it really is not. Intensity helps you in the EPL the same way it helps you in other leagues. If England is indeed more physical, then this is by choice, not by necessity. Every requirement to be especially physical is a perceived one, IMO.
Moreover, I don't find Sancho and Havertz comparable. Sancho had it incredibly difficult because United was pretty much the antithesis of the Bundesliga. Rich but no overarching strategy, no distinct style of play, outdated infrastructure and training philosophies, etc. etc. Chelsea was nothing like that and hired a top coach known for his tactical insight shortly after Havertz' arrival. Havertz also doesn't look low on self esteem either. With him, it is the next step in his development he needs to make, with Sancho the issues seem much more sincere since he looks several levels worse than in his best seasons.