I don't think anything is quite as effective as having a conversation with others. It challenges you in a deep, possibly mind-changing way that a book cannot.
I didn't quite realize this until, after 5 years of building things on my own, I had a co-founder. I feel like I gained 10 years of experience just from working with him and having to explain my solutions or why I didn't consider this other one, or having to realize that I was in fact wrong. That's a hard pill to swallow when you've only worked with books.
Craftsmanship is more than just learning new things. Sometimes you need to have your entire model of what's best swapped out for new ideas, like society realizing a republic is better than feudalism, than never evolving past your original position.
The steel man of that argument is that yes, with no feedback you have nothing to evaluate your decisions against, and all feedback is outside feedback.
That said, I think the person you're responding to meant outside feedback as in opinions from peers, which I agree is very powerful, but not strictly necessary.
Is it true? If I read book or blog, I improve my skills. Sure, feedback helps but I don't think that it's a hard requirement.