Is he really supposed to have a movement speed of 50 feet? Was this a typo? If not, where's this coming from? As a dwarf, shouldn't it be 20 feet (slow and steady)?
The only thing I can think of is that he's riding his fiendish giant wasp, but that his weight encumbers it a bit (reducing it's speed down from 60 feet to 50 feet)?
He has the impossible speed mythic ability from the Champion path. That increases your base land speed by 30 ft.
I didn't see that listed. I'll have to take another look. Thank you.
You can find it on the SQ section of the statblock.
Yea, I overlooked it. Sure enough: there it is.