This is talking about the case where you're just testing something out – prototyping it for a few branches before implementing it for the whole thing. In this case, _you_ are your own user, so it doesn't matter if you see the type errors.

If I’m making a potentially wide-reaching change, I’d want to test it against everything else so I know for sure that it’s actually viable in the context of every instance of its use.

Also, some statically-typed languages (like Haskell) allow you to defer type errors to runtime if you really want.