Big caveat that I’ve not read the book but I get that the premise is “legacy code is code that is untested” and I’ve always struggled with that definition. I think a more accepted definition would be that it is not well understood, uses old tooling/languages/runtimes and is usually high stakes code (driving some business that was successful enough to last longer than the staff that coded it up). I’m sure added tests help deal with legacy code under that definition but not sure it’s the panacea, I’ve certainly seen code without tests lots very high coverage that people area afraid to work on due to them being unfamiliar with the domain and codebase.