The force he would experience during take off shouldn't be any different than he was sitting on a chair in the plane, that's if the line is taught when the plane takes off.
As far as as arm strength needed, for you other observant people out there, you'd notice that the "kite" is directly tied to the aircraft and the handle for the rider to hold on to is just for stability and maneuvering.
If you watch the whole video, you'll see how this operation ends, by the rider release from the "kite", and free fall to parachute altitude, and the "kite" is still attached to the aircraft.
Yes, it's directly tied to the aircraft, but, wait, I'm not getting what you are trying to state by saying the handle is just for the rider to hold onto for stability and maneuvering. I... thought that was obvious?
Here's the thing: their's g-force during take-off, which as you point out, is not at all extreme. But then there's wind resistance, which you do not experience at all in the fuselage of an aircraft. That wind will be buffeting you hard, harder than any towed sport we've witnessed.
Now, it should be relatively easy--for most people who dare to take on this future stunt--to hold on. But I can bet many people would not be able to.
Think of it this way: try holding onto a bar during a hurricane, tornado, or straight-line winds. It's entirely about the strength of wind speed, and relative to your stationary body, your movement forward equates to wind pushing against you. Your grip and upper body strength become a major factor. It isn't at all like being in an enclosed space where it is only during acceleration and deceleration that one feels that force: when exposed in open air, air pressure is constant. Enclosed cabins of various transportation methods remove that force for us, so we only feel relative velocity (or something like that - my knowledge of high school physics terms is very rusty... the concept I get, accurate phrasing I may not. lol).
I think it should be an absolutely requirement to hold ground training inside of a to-scale wind tunnel. Have a padded room/floor with a "kite" or "board" tethered in front of fans that are simulating first take-off speed and then forward flight.
And like other tethered boarding sports, take off isn't that simple. Now, you aren't trying to "get up" at the same time like water sports, but it still isn't an automatic assured thing even though are you standing. A real human body that doesn't have a static default (like the scaled training dummy) is going to have to deal with balance. And if you let yourself get off balance by even a smidgen, unless the boards have an exceptionally stable profile that allows for this (or even has an auto-balancing capability), being off balance and unprepared = you and the board flapping around uncontrolled.
It does seem like it might be a requirement that the feet are anchored to the board so that inversions are more easily executable. Letting go of the bar may introduce enough force for the anchored piece to rip free, otherwise, as long as you hold on, your feet remain firmly anchored.