I think the original article is somehow lumping the notion of sandboxes in with 'walled gardens.'
One does not imply or require the other. Sandboxes are (almost always) a force for good. And you can run 'sandboxed' in a completely open and free environment with no need to let yourself be forced onto a captive platform or through a portal to do so. No need to even install anything. Just boot off any 'live' Linux distro and you're there.
I don't mind a sandbox if it's reasonable. Sure, protect the user, but don't be an idiot about it. e.g. Providing read access to a media library isn't too much to ask, is it? (iOS does not allow this last I checked.)
Walled garden... Not down with that.
Regarding that story by Stallman...
I once heard how science fiction eventually becomes science fact. We've seen it in our lifetimes.
I believe that the same goes for horror stories... And we're seeing it right now. All the terror of the dystopian novels is becoming more and more real.
When Stallman wrote that, he wasn't a story-teller. He was a prophet.