> and I really don't like how the author is so stubbornly in favour of unused-variables-as-errors

FWIW, they also have a goal to emit as much output as possible, even in the face of compilation errors. They have stated that even syntax errors should have the compiler exit with a non-zero exit code, but still produce an executable that will give you a syntax error at runtime. The point of this being to allow you to iterate quickly, but force things like CI to fail.

Why not just use compile warnings and configure CI with -WError (same like your release build).

It seems like trying to fix the world of undisciplined developers at the cost of a common use case (experimenting and temporary accepting warnings).

The Eclipse Java Compiler is similar to this, and Haskell can defer type errors to runtime. You wouldn't make production builds that way, but it's otherwise a perfectly valid mode for a compiler to operate in.