The idea of her 8-year-old walking a mile down their busy street alone
    and hanging out with older kids made her very nervous.

Yes because car culture is one of the two actual risks to free-ranging. The other is trespassing culture.

    A growing chorus of experts suggests that families have drifted into an
    equilibrium in which we overprotect our kids offline–for example,
    by not letting them walk down the street to their friend’s house...

Yes but when they get to their friend's house they're where they started, in an adult-curated box.

What they can't do is what kids did throughout history, until recently. They can't spend hours with their peers, in adult-free free-range time. They can't consistently experience the scenarios that kids need to learn complex social skills and mistake handling.

    ...and underprotect them online

When kids have nowhere to go, online is what is left. And adults being adults, they're thinking hard about how to get rid of that too.