Some specs like to claim things that are out of their jurisdiction. A format spec has no say on where that format is used. It's best to ignore such hubris.

And in this particular case it doesn't even make sense because the entire point is to round trip WTF-16. If that requires one "system" to communicate WTF-8 with another "system" (which is really an arbitrary boundary) then so be it. And anything that expects UTF-8 will need to deal with "invalid" data in a usecase-dependent way anyway.