You're assuming the "legacy" system is being replaced by a new one, which isn't the scenario being described in the link or the book. They cover approaches for continuing to safely evolve the legacy (i.e. untested) system.

Rewriting the legacy system is one of the valid paths to maintain it. There can be multiple reasons to do that like operational risk which cannot be mitigated, maintenance overhead, risk related to the existing dependencies of the system.