In short, children who don't learn to read or write well in school almost certainly would not learn to read or write well when unschooled, but most unschooled children who don't learn to read or write well do learn perfectly fine in a traditional schooling environment.

Schooled children who don't learn to read for different and harder to deal with reasons, a quick top 3:

1. English as a second language (ESL)

2. Learning disability that was never addressed

3. Severe behavior problems that interfere with schooling (this is often caused by problems at home)

These problems are exacerbated by parents who are poorer or themselves ESL.

Unschooled kids, conversely, tend to have relatively affluent parents who speak fluent English. Overwhelmingly, the reason they can't read or write is that either no attempt at teaching was made, or it was made with no reference to educational theory. This failure mode is much rarer in schools.

P.S. I don't want to give the impression here that I think the US education system isn't a tire fire or that there aren't different pedagogical approaches that might be radically better for most children. I'm just saying I have nearly a decade of experience showing overwhelmingly that "unschooling", even when performed by some of the most educated and affluent people in society, fails overwhelmingly compared to even the sub-par experience of standard schooling in the US.