Imagine if methods on your C++ classes had to be called like method(object) instead of object.method(). While it's a meaningless syntactic difference, the latter makes autocomplete in an IDE possible.
I don't think visibility is important, since you want autocomplete to work both in public and private methods. It would be a crappy IDE experience if you couldn't autocomplete on private methods within other private methods.
And sure, multimethods are too complex for naive autocompletion. But it's useful for people to have tooling that can say "given an input tell me what outputs are possible" because that's how we write code.
But method(object1, object2, object3, object4) is different than object.method().