At a higher level, just about all the kids who have a shot at making the USA national Olympiad team use the most advanced AOPS classes as well.
The RSMs in my area cater to a large clientele who want better math than what the PS offers (gifted programs have been cut for the past several years). They feel that the rigor and the content makes up for what the PS education in math lacks. There are also many kids that go to RSM so that they can excel in the state standardized testing - the scores are used for pullouts in school for math differentiation. There are others that use RSM for remedial math.
The ones going to the national Olympiad teams have other avenues available to them. So, it is entirely possible that the tutors in RSM have not heard about AoPS.
