Balin,
I haven't seen the video, but from what others have said Larry is 100% WRONG!!!. They have said he tells you to turn the screw LEFT until it breaks. And I bet you did this if you watched the video. The correct way is turn the screw RIGHT as though tightening, which you are, until it breaks.
If you do it correctly, when the screw head breaks off the sight slide is all the way to the right. Then all you do is PUSH the slide all the way to the left of the sight and the screw and nut will protrude from the left side of the sight base and all you need to do is use your fingers to unscrew the windage screw and "slide out" the slide to the right.
If you do it the way Potterfield tells you to, you end up with the broken screw, the slide all the way to the left with no where to go, and the nut jammed firmly in its' recess in the sight base. Sound familiar? He "may" have changed his video, but I doubt it. If so you simply "screwed up".
You MAY be able to use a spanner screwdriver bit to turn the nut, but will probably just break the bit! The screw will be TIGHT!!!
In case you wonder why I know Potterfield is wrong, I have been changing these things for probably nearly as long as he has been alive! I have done hundreds of them.