I was excited to see how the integration works. The link proves it's packaged. Is it actually integrated somehow or is it only packaged at this point?

there is at least one community-managed flake to connect a NixOS machine to an OpenZiti overlay network, but i don't think there is an official NixOS module yet:

github.com/rochecompaan/openziti-nix

if your focus is trying it rather than distributing consistent versions to many folks, you can also use the ziti-edge-tunnel binary, define a systemd service and add identities (a given app or device can belong to (n) OpenZiti overlays via (n) different identities).