I would still consider CSP, Petri Nets, and Pi-Calculus mathematical enough to be wrapped under a mathematics price if they're influential enough. The first true computer scientists were mathematicians, and I still feel that much of the theoretical work in the field is closer to "mathematics useful for computers" than its separate field.
In the spirit of the nobel price, "mathematics with the greatest humanitarian impact" leaves plenty of room for the inclusion of influential pieces from theoretical computer science, especially as those prices within mathematics that do exist already include loads of mathematics that require computers to prove or solve.
I guess, but they certainly feel categorically different than something like Runge-Kutta. They're more about the study of algorithms, which is generally where I've drawn the line of "computer science vs math".
I'm not sure I agree with that. There's plenty of theoretical computer science that isn't really "engineering" and would fall into a pretty different category than stuff like FFT or Navier Stokes.
If you look at something like Concurrency Theory, for example, and work with stuff like Pi Calculus or CSP or Petri Nets, those aren't "engineering feats", but also kind of fall into a different category than the rest of math, or at least pretty different than Navier Stokes. I think you could make a pretty strong argument that CSP has been a pretty big innovation in regards the academic state of the art while not simply being engineering.