The key is that the coordinates go from (0, 0)
in the upper left corner to (1, 1)
in the lower right corner.
You have the quote from the video just a little bit wrong, and I think that's causing the confusion. The correct quote is:
The StartPoint and the EndPoint give the coodinates on a 0 to 1 scale of the start and the end of the gradient. The 0.5 and the 0 for the start point means this gradient starts halfway across the control horizontally and at the top vertically.
So the coordinates go from (0, 0) in the upper left corner to (1, 1) in the lower right corner.
When you first create the control, the gradient starts at (0.5, 0) and ends at (0.5, 1) so the gradient moves from the top middle to the bottom middle.
In the video I change the gradient's coordinates so it starts at (1, 1) and ends at (0, 0).
I hope that helps. Let me know if it's still unclear.