I'm sure various arguments can be made for whether or not that's true of different people (and teachers), but I grew up in a pretty mediocre school system and still had very smart teachers teaching cool classes that were so, so good because of their very specialized knowledge. I can see how K-8 might be much easier to replace with homeschooling, but I have a hard time with secondary school.
If my mom had taught me high school chemistry, for example, she could have gone through the book and lessons with me with minimum difficulty. But I had a chemistry teacher who'd left being a college professor/researcher so he could be back with younger students, and he was amazingly smart and fun to learn from. I'd have missed that completely.
The author of the article comes from another angle entirely: It's a situation where the high school teachers' specialized knowledge isn't nearly good enough. I get that too. If I was a quantum physicist, I'd bet dollars to donuts I could cook up a better physics student than your average college-prep science class -- but that doesn't mean I'd be better at everything else that happens in high school, too.