That dev told me:
This too is to abstract the test from things i don't want to really test, that sould be tested in the calendar's model test. Abstracting from others components and models behavior is especially useful when you don't know ALL the behavior of everything in the app, and because that behavior shouldn't be tested in this model, so you save time by not having to know how that component/model exactly works and by not setting a lot of enviroment variables.
And I answered:
You are right, if writing unit tests is what you want.
I'm asking, please, do not, go further, and write tests that assume less.