In their charts, they measure stand-over from the ground to the height of the top of the head tube.
Assuming size M and 55 mm gravel tires:
Wheel radius = 622/2 = 366 mm
Tire height = 55 mm (will depend on the tire, of course)
Standover height as defined by Mondince = Total tire radius - BB drop + Stack
For a size M, that is (366 + 55) - 80 + 574 = 860.
In their charts they say 863, so I'd say that's close enough.
The thing is that most manufacturers don't measure stand-over height at the top of the headtube, but somewhere in the middle of the top tube. When the top tube has sloping (as it's the case here), the real stand-over height is quite a lot lower. In the middle of the top tube it'll be somewhere around 800 mm