Thanks for sharing the helpful video. I think the confusion here is that the Large Desktop media query targets screen sizes including and above 1200px, not below (like the other tabs). It uses min-width rather than max-width in the media query code. The dark shading on the top ruler shows the scope of each media query.
It’s sometimes useful to apply styles that effect large screens (e.g. above 1200px). But if you want the styles to affect any screen size, you can use the All Devices tab. Or, if you want it to affect the range below 1200px, you can edit the media query code via General > Edit media queries. Just change min-width to max-width for the Large Desktop media query.
I’m aware that beginners find the mixture of min and max width a bit confusing, and so version 7 will have max-width only by default, and make the direction (above/below) more explicit.