Not necessarily disagreeing with you, but moving to the NL lowers his raw xFIP quite a bit too. We can look at xFIP-, which is his xFIP relative to league average (set to 100). It's also park adjusted. So rather than Hendo having a massive improvement of 4.42 to 3.49, he went from 107 (to 105) to 94, which is still very good but not ridiculous or anything, he went from a bit worse than average to a bit better than average. Using xFIP- helps factor in contextual stuff like having to face pitchers for example. Again, not disagreeing with you.