This is cultural bias spoken as though it’s universal
Plenty of fulfilling lives out there that don’t include home ownership, being a parent, having secondary schooling or being partnered with offspring.
Plenty of fulfilling lives out there for any conceivable metric, it doesn't mean that the article is incorrect.
For me, home ownership nor children would be part of my “ideal middle-class existence” quite the opposite. I want stable housing, I don’t necessarily want to own it and all the burden that comes with ownership, and I definitely did not and still do not want responsibility over bring a child into the world. Thankfully, my wife agrees.
If we were to even approach a definition it might be something like “Do you have stable housing, access to nutrition, hygiene, transportation, etc. while also having enough capital resources and time to achieve your personal goals?” And that’s going to look different for everyone.
Most people are not well off and anecdotally I also don’t know many happily married couples nor happily married parents.
Directionally, humans want stability before trying to create offspring. This is the whole r/K selection thing. And even though what constitutes stability is going to vary wildly based on culture, expanding the scope to "present & past" is more of an exploration in what constitutes that stability rather than a repudiation of the main point.
Congrats.