Maybe this comparision will make more sense.
You’ve seen the moving walkways at the airport right? Basically a giant treadmill. Now put a handrail on both sides of the walkway and turn the walkway off. Put on a pair of roller blades and see how fast you can move along the stationary walkway using just your hands on the hand rail.
You can picture this right? Guy on rollerblades pulling himself along down the stationary conveyor, wheels spinning. Lets say he can go 5 MPH.
Now, as in the airplane question, we’re going to turn the conveyor on and have it match his top speed of 5mph. Remember, he’s not pushing with his feet, he’s simply using his arms to pull himself along, the same way a prop pulling the air would move an airplane.
Ok, so start off with our rollerblader simply holding the hand rails while we ramp the conveyor up to 5mph. You agree that he’s stationary right? That even though the conveyor is going 5mph the fact that he’s on rollerblades and holding on to the railing that isn’t moving makes him stationary right? So what happens when he pulls himself forward? He’s moving right? His arms are still capable of moving him at 5MPH, the treadmill is matching that 5mph going the opposite direction. End result, guy going 5mph through the airport, treadmill going 5mph, wheels going 10mph. Maybe he only his 4.95 MPH because the friction slows him down ever so slightly, but he’s still moving forward.
Now stop making him use his hands and put a big gas powered fan on his back. Do you understand that he’ll still go forward, just as he did when he was using his hands on the railing? Neither his hands nor the fan are pushing on the moving conveyor so they are not effected by it.
Finally, replace the guy with a fan on his back with a plane. It will still accellerate to takeoff speed. Lets say 50mph since Mythbusters is going to use an ultralight. Plane going 50mph one way, conveyor going 50mph the other, wheels going 100, plane has enough airspeed to take off.
If you don’t agree with any of this, exactly where did I lose you?