I think Lemmy is correct here and the Mozilla docs only refer to the text content, not elements. It also mentions that < and > still have to be escaped to be displayed.
The HTML spec https://html.spec.whatwg.org/multipage/grouping-content.html#the-pre-element defines that the element contains „phrasing content“ which \ is part of.
It even explicitly mentions nesting \ inside .
Same applies for Matter devices. They still might provide additional cloud-based features though.