The survey isn’t about the “niche” subset of the population that is religious. It is about the composition of the entire population. Not a subset of the population so that isn’t relevant.
Edit: to be clear, I understand that there may be some niche subsets within this survey that may not be represented because there are only 20 people in the US that believe in that weird religion, but again, that has nothing to do with the larger, non-niche subsets which are absolutely represented with enough accuracy to draw statically significant conclusions with a sample size of 1000
Even within those services you could also just have an actual library that functions the same way as an iPod. You can create playlists or just play your entire library if that’s what you want. There is no need to interact with any algorithm in order to use these services.