I think people say IFR has nothing to do with the weather just to try to sound cool and confuse the unwary. IFR isn't defined by the weather. It's also true that you can elect to fly under IFR regardless of the weather. But neither of these facts means that IFR has nothing to do with weather. If the weather is (or rather causes you to be in) IMC then you must fly under IFR. In this case then, IFR has a lot to do with the weather.