This wasn't enforced or known until fairly recently [1] and finally got its public attention during pandemic. Those bad PR was what caused Apple to make concessions. Basically since 2018 Apple has been steadily Pushing these changes for their Services Strategy revenue target. If you look around on the Internet or places like MacRumours, you find a group of "Apple Apologist" who are perfectly in flavour of Apple charging 30% for these educational digital class.

>but not when teaching to one person interested in guitar for example?

Teaching one to one are exempt from sign up link. Which is basically a FaceTime Call. But that is only on the basis if the teaching is "live". If you are showing large quantity pre-recorded Video you might / will be counted towards "Digital Goods".

But again Apple Apologist thinks it is Apple's platform. They can do whatever they want.

Edit: I wanted to add some additional context. A lot of these were rules not previously enforced before 2016 / 2018. So App Store has been perfectly fine for many conducting online / digital business. And the App Store problem isn't just about the 30% commission.

[1] https://www.cnbc.com/2020/11/23/apple-extends-fee-waiver-for...