You seem to conflate understanding what a module does with measuring the complexity of it's API, but those are two different things, and they can be discussed and analysed separately. I also don't agree that measuring complexity is hard - there can be many measures - and some are rather simple, for example you can just count the number of objects and methods in the API. And even such a simple measure can be practical - if we could show that all other things are equal (which can be hard or impossible, I agree, but doable in some cases).