> it's not very easy for us to say, "just turn off the type checker when you use our library" :)

Sure, and that is a problem with exposing certain kinds of dynamism when your customer base is the kind that is inflexible about typing.

OTOH, a lot can still be done with dynamism on the inside and a typed public interface, and also there's a significant part of the community that accepts selective disabling of typechecking as an acceptable when dynamism provides adequate benefits.

Mypy is scoped to have different levels of checking per package. Normally it doesn’t scan third party libraries beyond the signature of public functions. See the follow_imports options.