> The strictness even allows us to remove the if check inside the function, since now TypeScript gives us the compile-time guarantee that obj will always have property key.

This is a dangerous suggestion. While the author does acknowledge it is a compile-time guarantee only, that doesn’t imply it is safe to remove the if inside the function.

An API call, reading a file, calling less well-behaved libraries or making some system calls can all result in objects that pass compile-time checks but will cause a runtime exception or unexpected behavior.

As for the thesis of TFA itself, it sounds quite reasonable. In fact a high “level” of typing can give a false sense of security as that doesn’t necessarily translate automatically to more stable applications.