Hey
@ed lespaul , that's ok, I only really referenced to it in the lesson, but you're right. What you describe in your post is known as "derivative" or "relative" modal theory. You describe it well, and it warrants its own (longer!) YouTube lesson.
If I was working with a student, I would have covered the derivative approach first. This video is a few lessons on, and rather than looking at where modes come from, it's about exploring the individual mode in greater detail, and you can only do that by understanding it's individual qualities, in this instance utilising "parallel" modal theory, where everything is referenced in relationship to the key centre (A in the example). It's not a case of choosing sides (eg CAGED Vs 3NPS - a ridiculous argument for another day!), it's about expanding knowledge to focus on individual modal qualities.
Hope that helps explain the approach - I'm certainly not one of these snobby guys who says the derivative approach is wrong, this is more about developing beyond that foundation.