Yeah, sure, slightly different models. But plenty of people are selling this kind of content and achieving financial success -- and that's without the benefit of MDN's brand recognition. Clearly, it can be done.
What is your point now? That these counterexamples don't have the exact same % mix of free vs. VIP content as MDN is proposing, therefore MDN can't financially sustain itself? Seems like quite a supposition. I see a wide variety of free/VIP % mixes out there. Doesn't seem like there's one right answer or magic formula.
MDN may or may not succeed with this model. As with most things it comes down to execution. It's relatively new territory for Mozilla so I'm not convinced they'll pull it off. But it's not like some wild and untested/disproven model.
It is absolutely possible to make money selling classes/tutorials (e.g. via Udemy or any of a variety of other online class platform), but MDN is already committed to making its classes/tutorials available for free https://developer.mozilla.org/en-US/docs/Web/Tutorials
It's an essential component of MDN Plus that they have to say exactly what they say in TFA: "Nothing is changing with the existing MDN Web Docs content — this content will continue to be free and available to everyone."
As a result, MDN can't practically sell books OR classes. What's left? VIP access.