>I don't get it, so no company can provide a walled garden? Or at least, not if they're successful?

Framing things as you have leads to the impression that, by analogy to oppressing a person, restricting a large company is bad. But a company is not a person. Companies are legal constructs that exist because they are useful to society. A monopoly distorts markets and stifles innovation by disadvantaging potential competitors. This is not beneficial to society and so should not be permitted. So yes, a sufficiently large company should not be permitted to do things that might be permissible for a smaller company.