It sure is!
Somehow I had learned that CRT lines persist, and the eye persists, and all that together unifies the lines into an image. This shows that the entire screen is dark except for the line that's being drawn (and maybe one or two before it) but not the entire screen.

I wish he had addressed the fact that our brains modify the optical input somewhat. Imagine an image projected onto a wall painted white. There's enough ambient light to see that there's a wall there... and that's the darkest black you'll ever get in an image. But when you actually project an image up there, black places inside the image look darker than the wall surrounding the image. Bright places make the unlit portion of wall look darker than they are!