What I don't understand is that a person would not just ride away from this what looks like a relatively slow moving phenomenon and in ultimo just release the kite if this is coming too close. I don't have experience with this type of weather and so I probably miss something.
The phenomenon is moving slowly but wind towards it ain't. Your wind window will be wrapped around the spout or on land dust devil as from every direction wind surges towards it and escapes only from the top.
It is the same as with real tornadoes. People try to outrun them (if they get close) with car capable of doing 150km/h and thinking I am fine as tornado moves 30km/h. But when you try to outrun it your head wind might be 200km/h and your car cannot win it due to lack of HP required to win the wind resistance.
F0 wind speed: 40–72mpg 64–116km/h diameter: 6–17 yards (5.5–15.5 m) Light damage.
"Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged."
So apparent wind affects there as well, it is moving speed of phenomenon plus wind rushing into phenomenon plus your movement vector. One some directions they cancel each other and other directions enforce.
It's hard to punch out while you get lofted. So if the wind is 70 knots that means in 1 second you are almost 40 meters in the air. It's nearly impossible to go from just riding around not suspecting anything, feel the power of the kite lift you and be able to punch out in milliseconds. You must punch out before you get lofted. If it snuck up on him then there's not much you can do. The problem is that after you get lofted by a dust devil or waterspout, after get you get out of it, you don't simply float down, because there is another wind that is rushing straight down to the ground so the kite has no lift after the lofting ends. If you see a water spout, remember to use your whistle and warn other kiters, make sure everyone flags out. The way things happened, I dunno if there's much different he could have done. My condolences to the friends and family.