By walled garden I think they mean the servers. Because the app isn't a walled garden. It is open source though centralized (Moxie has argued extensively about how this allows faster development. Though ironically Signal is known for slow development). But the servers are also open sourced so there's nothing stopping people from creating private (or even federated) "Signal" apps. You just couldn't call it Signal in name. And I don't blame Signal for keeping their servers to themselves. They're not a data center and already running a tight ship.

I've never really understood the argument. Just because someone hasn't done something doesn't mean it is a walled garden.