I don't want to give you the wrong answer so I've forward this question to the SSBC network for one of the core developers to answer better.

I'd be surprised if there was no security for pushes. The repos I've worked on did require an invite from the creator.

Marak It sounds like you are working with private repos. With git-ssb currently a repo is either public or private. Private repos are encrypted to a fixed set of recipients so only those keyholders can access it. Public repos are unencrypted.