These "what ifs" are kinda funny because the origins of JSX can be traced back to Facebook's XHP[1], which took explicit inspiration from E4X[2], an early JS standard that looked and behaved similar to the library described here.
[1] https://engineering.fb.com/2010/02/09/developer-tools/xhp-a-...
I'm a self-motivated adult learner, so I don't know what it's like for kids. Though the program was originally designed for them, so I suspect their experience would broadly be similar to mine.
As other commenters have mentioned, you need to be okay with grinding through problem sets with no videos or UI pizzazz -- maybe this doesn't work for everybody. I'd compare it to the difference between trying to learn a language through scattered YouTube videos and Duolingo versus tandem and grinding on a good Anki set.
NB: I'm taking it for the Math for ML track and am currently most of the way through the Math Foundations III course. So I can only comment on the lower level courses.