I don't think this is true. Granted, last time I tried to get good at an RTS was toward the end of the Brood War era but the established wisdom at that time was very clear that hour-for-hour, time spent practicing resource management was much more effective than time spent practicing clicking quickly.

Yes, really good players click fast, but they also have impeccable resource management. The group I played with did run the obvious experiment: the best one of us was forced to play against the rest (one at a time) with an artificial click frequency limit. He felt like his abilities were greatly reduced, but he still beat everyone else quite easily.