That certainly makes sense. There MUST be a system for replacing lost fuel volume. But what I don't understand is how it lets air in? I clearly see the "out" mechanism? I am always fascinated by simple effective, and usually older, engineering.
In any event my cap seems to be working. However on occasions I have opened the cap a felt what I would consider to be too much pressure. I have not associated this with poor performance. Only noticed when getting gas.