Did you actually try scrolling through a long X settings sections in Edge? You’d see what I mean if you had.
The main usability issue is not the width of the scrollbar (I don’t believe anybody is actually trying to grab that anorexic thing). The main issue is that due to the replacement of a classic scrollable area with this “virtual” area, all the usual scrolling interactions are broken in Edge. I can’t use my touchscreen to simply drag the area up and down, nor can I use mouse wheels, or trackpad gestures. The browser simply does not know that the area is supposed to scroll (as it shouldn’t - the scrolling is being faked with draggable elements and event listeners). This should have been fixed with a polyfill and graceful degradation.
The ONLY way I can scroll in Edge is either try to grab that skinny bar (nearly impossible on first try) or, my go-to workaround, find a text field I can see, give it focus, and then use Tab to get to the hidden fields below the screen.