Depends on what you mean by "real money."
On the fully indie side of things Michael Hartl has been doing it for a while with Ruby, front-end technologies, and seems to be doing OK for himself. I don't know what kind of profit we're talking about but it seems that it's enough to keep at it. https://www.learnenough.com/
Then there's Udemy. Max S. has 1.4 million "students" which I believe means his courses have been purchased a combined 1.4 million times. If we assume an average selling price of about $10 that's a lot of money even when we subtract Udemy's cut of the money. https://www.udemy.com/user/maximilian-schwarzmuller/
In general, be thoughtful when considering what you think "makes money" on the web. While not easy to achieve, there are a lot of folks making very good livings selling things on the web. But of course it's not going to make headlines if they are "only" making $100K or $500K. The press is only really interested in gaudy public stock offerings worth billions.
In the specific case of MDN, it certainly seems like they could sustain themselves using this model. If 10,000 people sign up and pay $10 a month that's $1.2 million/year in revenue. Certainly enough to pay for a rack of servers, some dedicated staff, and a rotating cast of outside contributors to write the articles.