The important parts here are (1) that they remained metrics driven and (2) that they focused on repeatability.

1. Both TypeScript and Flow are gradual type systems. They both explicitly allow escaping the safety and coverage of a type system, either to allow easier interop with existing JavaScript packages or to allow existing JavaScript codebases to incrementally adopt types. So there are always holes in which errors are reported, regardless of whether they had to silence some explicitly or not. By measuring the extent, they have a yardstick against which they can drive further type adoption.

2. Repeatability is the only way to do large code migrations. Write the steps in a script, run it, find the bugs in the script, fix the script, and repeat. With 300k lines of code no doubt they have dozens or hundreds of people committing daily. So a script is the only way to sneak in during off-hours, get something in that doesn’t race to conflict with someone else, and land the change. A long running, manually crafter branch here would not work—it would effectively operate as a fork of the codebase for the entire duration of the migration, with all the downsides that a fork entails.

Kudos to the team; this is a very impressive feat!