(I'll admit that it doesn't address the point you were trying to raise rather ungraciously, namely, how well NixOS deployment tooling scale to really large deployments, which is a good question to ask).
My hunch is that it could scale well, but would require some integration work in order to use it nicely with existing orchestration tools. I don't know exactly what that would look like, or if it would be worth the effort of going off the beaten track.