How is it easier to change a service interface than an API within a single app? The former requires a specification and tooling around serialization/deserializ

How is it easier to change a service interface than an API within a single app? The former requires a specification and tooling around serialization/deserialization to have any meaningful guarantee of correctness, it requires error handling to reap the benefits of decoupling, the latter can be covered with a unit test. I'm sorry but this is pure hype to say that microservices make refactoring easier: that is true if and only if your boundaries stay static. It is a huge assumption that anyone can draw those boundaries correctly for a large microservices architecture in its infancy.