Not a silo, it's a federated system, and actually federates with pre-existing GNU Social servers and other OStatus-using services, besides the different Mastodon servers.

In the nearish future, it will also federate with Pump and other ActivityPub adopters, too (Diaspora might be in there).

Also, it's not possible to federate with Twitter.com, that's a given. Now that's a silo =)