Thanks Ron for the quick reply - like the original poster of the question I have wondered about the screen reader behavior for these.
Being able to see the layer content along with the base layer content in the focus order, **and** being able to order layer content above base layer content, does create a bit of ambiguity if you don't have access to a screen reader to actually observe the behavior.
While the article is helpful (I've read it multiple times) imo it could benefit from adding some clarity about how reading/focus order for layers actually works in a screen reader.