Well, they didn't really need to. A complete rewrite is effectively a different project. You may feel comfortable using a new project in prod, but most people are not.
Project A: used in production for 3 years - high trust.
Project B: Has yet to be used in production - low trust.
IDGAF about automated tests, let other users shake out the inevitable bugs that show up in prod and after a few years of stability, then we'll see.
To me, it's not about whether humans reviewed the code or not (they didn't), it's more about "here's this brand-new shiny codebase of ~1m Sloc, of which exactly zero lines has been used in prod".