Calling the whole article "just a hidden ad" is a bit of a stretch. Sure, it might attract interested parties, but imo the reference to their community is more of an example than an ad. In the same section the author recommends building your own and wanting to keep the community small.

I write about community from lived experiences and reference my lived community experiences, it's the most authentic way to do it.

I'd actually prefer people don't join that community. It's a very special little place on the internet for me. And people that know me know understand it's got nothing to do with money, that it's a fun experiment and the money is just a way to attract the right kind of people and make the community a joy to run.

I don't know... The article does look like a veiled attempt at advertising their own community where they get to earn a $99 fee for every new registration. What I felt was in poor taste was that this motive of advertising and profit was hidden until the end of the article. Not calling it a hidden ad would be a stretch IMO.

It's an article about a community site about building communities. Not calling it an ad is a stretch

Yeah, I think that if this is the conclusion, at least partially, you might as well open the article with, "I have started a Community starter hub for people doing X and I want to explain why I believe it is useful"