The acceptable anchor requirements state that in order to be an acceptable anchor, it is required that:

possible anchor is in scope for positioned el, per the effects of anchor-scope on positioned el or its ancestors.

It doesn't seem to make sense to refer to anchor-scope on positioned el itself. That would mean that positioned el can only reference itself or a descendant, neither of which makes sense.

Perhaps change the text "on positioned el or its ancestors" to "on any ancestors of positioned el"?