Where exactly does it say that you can't start by actually learning how Rails works, before you build things with it?

I think the biases of people who were experienced big system developers before they moved to Rails, are very different from those who started with it, potentially not understanding web app development very well, and then produced something challenging to maintain

No, I realize that. I was thinking more about complaints along those lines in general rather than anything you said specifically. Maybe it's just a case of setting unreasonable expectations as a consequence of how Rails in particular is recommended to newbies.