I wouldn't say it's confirmed the D-loop is the issue - but it's a solution. If the peep turns for any reason, the D-loop can re-orient it to the shooter when drawn.

Strings stretch, peeps turn. I've grown up with horizontal peeps, such no matter what my string does, I don't have to fight rotation. I sacrifice a little for shape and a little brightness loss compared to an angled peep, but it's never bothered me, and never seemed to grow my group size (carefully matching diameter with sight housing diameter).