I imagine it would be very difficult to teach a topic no one is familiar with by teaching how to test it _first_. A TDD approach to teaching a brand new topic would be very hard to follow and require the viewer to go back to re-watch the testing portion. (Imagine teaching a total beginner how to test a function before teaching them what a function even is...)
However, I think there's a lot of validity to the idea that every tutorial should include lessons on how to test it that new topic, and even give examples of how to implement a TDD approach. But those lessons would (IMO) have to come after the initial parts of the tutorial that taught the concept first.
I find this ^^ to be one of the main selling points. All online courses are in danger of falling into the "Tutorial Hell" category, where they spend all their time teaching you without the student actually being more capable afterword. The Scrimba platform significantly lowers the barrier for students to get their hands on the code and actually obtain new skills. I have yet to find another platform that lowers that barrier as efficiently/well as Scrimba.
It's a valid point - if I were concerned that Jr. developers would parrot every line from the course to the ridicule of very particular Sr. developers I would have specified that the Virtual DOM makes React faster than if it were trying to directly use the actual DOM, all else being equal.
But since that wasn't a concern of mine when I made that video (it is far from the main takeaway of the course), I kept it as simple as it needed to be to get the point across.