That appears to be for consumers of federal government web services, which I would not expect to prevent large breaches. The service itself would be a massive single point of failure, and I'm not sure how it would prove a contractor has ZTA, or whatever an administrator thinks that means, all the way down.

It alone is not a silver bullet for ZTA, but authn and authz are critical components of ZTA. Most are not good at auth*, should not roll their own, and absolutely should delegate to a service that provides this functionality (commercially, you see this with Okta and Auth0, for example).

While primarily targeted at the public, login.gov is used for some internal gov sites as well.