Plasma Mobile uses a different KCM for power stuff afaik. On desktop, I wasn't even able to resize the window to this narrower width with regular means, I had to use Window Rules to get this to show.
LGTM either way.
The rest of the layout is a straightforward Kirigami.FormLayout, so theoretically it should be able to collapse to narrow mode. I figure the Flickable gets in the way. Let me see if I can fix it.