I think he's saying though that are a lot of home schooled kids who do have a lot of friends. If it is done well the parents will have enrolled their kids in extracurricular activities, at the very least to satisfy the "gym" requirement, where these kids can socialize with other children and make friends. Socializing with other children their own age is a very important part of education, its not what people think of when they picture an education (you more tend to think of the classes themselves) but its just as, if not more important. If they're not getting the opportunity to socialize enough the parent is not "doing well" in the slightest at home schooling.
Some parents decide to home school not because they want to isolate/protect their child but because the local school district sucks complete ass and there are no private schools in the area, or they can't afford the tuition. I'm talking like a real shit school with textbooks missing 1/2 the pages, teachers who don't give a shit, etc. Or districts that refuse to teach certain topics like sex ed or teach on an "abstinence only" policy (basically a government approved lesson plan that outright says "if you have sex before marriage, you are a worthless heap of shit that should live in shame for the rest of your life"), and/or teach that the Civil War had nothing to do with slavery, or gloss over slavery entirely.
They feel that with the proper textbooks they can accomplish more than what the school can. In some cases they're right. I would not suggest most parents attempt high school level course work, if the parent isn't 100% confident in the subject they're teaching they aren't gonna do a good job; but I think a lot of parents are intelligent enough to teach elementary school level history, science, math, etc. to their child.
Again though socialization is incredibly important and if the parent isn't going to put the time in to make sure the kid gets that socialization then they are deeply effecting their child in a negative way. There are places though for kids to meet others of their age and make friends outside of school. Like at the park, or being enrolled in an extra-curricular sport