Typing adds constraints - the type must be represented by and implemented in a particular type system. A trivial anti-modular constraint is that interacting modules must all use the same type system. The type system matters too - in Java, choosing from easily implementable types can easily add a lot to the complexity of the data structure, leading to a complex interface (in a more expressive type system, less of this happens).

More trivially, adding types IS adding constraints that are verifiable at compile time.

If compile-time verification of that sort is not essential (or not desired, which is the more general form of not essential), then the constraints are non-essential (or not desired).

---

Whether you choose to

1. Choose a particular data-structure and implement both modules to pass and receive that data-structure.

2. Specify an interface: The interface requires the data-structure to have certain properties (e.g. some methods work on them). The modules then build to the interface.

3. Specify that an adaptor exist to convert the data-structure to whatever the module requires.

is a somewhat different issue, although the kind of type system used does affect the implementation complexity and effort required.