This is true in a useful sense. The more you know (about anything worth knowing) -- the more you realize you've yet to learn.

The more holes and short-cuts you're (painfully) aware of in tools and libraries everyone use every day, to get stuff done.

To be aware of how artefacts are made, is to be afraid. Be that artefact sausages or firewalls. The trick is to then learn enough to know which sausage to eat: or at least find a level of acceptance that lets you enjoy the stuff that tastes good, even when you know how it's made.

And after lunch, maybe you'll serve a json rest api with php.

I'm a generalist myself. I've become decent at a wide-ranging set of skills - cooking, feminist lit-crit, Arabic drumming, odd things like that. I've become a good but not awesome expert at a few things - configuration management, guitar, and others.

An important exception to this entertaining aphorism is observed in the Dreyfus Model of Skill Acquisition. Becoming an Expert (in the five level Dreyfus model) in something makes it much easier to become Competent or Proficient in other things. This has certainly been my experience, as things I learn in one area apply to learning others.

The result is that people who never commit to expertise in anything struggle to achieve proficiency, or even competence in anything. They're neither generalists nor specialists... they're just trapped.

Very true, like with most things in life, balance is key. Full stack developer who's emphases is X is a grate candidate, assuming X is what you are looking for. Full stack developer who is "great" at everything is probably not so much. My personal attitude in life is to know as much as I care about subjects which are my core competency and are most interesting to me, and know enough about other fields to be able to competently hire an expert in those fields, and be able to spot instances when that expert is bullshitting me.