You need to have a lot of understanding to make sense of that test. Don't expect any bike with a 250w hub-motor to behave the same.
The main point is that his motor maxes out (not limited) at 25 km/h in a 20" wheel, which means it's a 230 rpm motor. That is very slow for a motor in a 20" wheel. That is a typical speed for a 250w hub motor that's used in 26" wheels, which would have about 75% of the torque and would stall out on that steep hill because it would be running at lower rpm out of its efficient zone.
If you want good climbing ability, any small hub-motor can give it if you use small wheels. As I've said before, even the tiniest motor (200w Q85) can get a 100kg rider up a 15% hill without pedalling if you put it in a Brompton with 16" wheels.
You don't need much power to go up hills slowly, but most hub-motors are designed for faster speeds, which means that they have very low efficiency at low speed, so can't produce the power when you need it most. If you choose a motor with the right maximum RPM, you can get full power for climbing.
As an example, take a typical 26" wheeled bike with a typical 230 rpm motor, typical 15A controller and 36v battery (40v charged). A steep hill can drag the speed down to 6 mph. That's 30% of its maximum RPM, at which point its efficiency will be about 45%, so you get 15 x 40 x .45 = 270 watts of assistance.
The same motor in a 20" wheel at the same speed will have an efficiency of about 60% and give 360w of power, which is 33% more.