Thats true to a degree, but the distinction makes sense to me because angular is not the only project using reactive concepts.
RxJS is just one of the implementations [1] and people that haven't grokked it would struggle with other project utilizing it as well - even if its a React frontend, Go backend etc. (and as others have pointed out: angular should not be considered a poster-child of reactive programming. they made it work somehow, but its really easy to get memory leaks even if you're just copy-pasting code from the official angular docs)