I think that is a matter of perspective. Operating systems wholly reliant on package managers are arguably walled gardens, since acquiring and running software

I think that is a matter of perspective. Operating systems wholly reliant on package managers are arguably walled gardens, since acquiring and running software that only exists outside the official repos can be a hassle. But I don't think most people mean either of these things when they say "walled garden". They are referring to ecosystems where the user's only choice for installing apps is an app store owned and maintained by the OS vendor.