I think that the text in the original post outlines the intended distinction between the two tags quite well.
(While I do think the text can be used as-is, I would personally go beyond saying that highway=footway + area=yes polygons “will not necessarily always” have highway=footway ways running through them. They should usually not have highway=footway ways running through them. Drawing those ways is mapping for the router due to the current lack of support for navigating over areas. Which is perfectly understandable, but we shouldn’t forget that it’s a temporary hack.)
It would of course be good to refine the definitions of when and how to use these tags once we have input from data consumers using them for real-world use cases. But people who are currently mapping areas need to base their choice of tags on something. And the proposed clarification is both aligned with the original intent of the tags and also seems like it shouldn’t impede future use cases in either routing or rendering.