Mathematica borrows heavily from FP and is well known for its computational performance. And yes, a lot of its guts are imperative, but most of the internal libraries that encapsulate the actual mathematical concepts are written in Mathematica itself.
Those guts that 'actually perform numerical calculations' as referenced by the above post are not only imperative, they're in Fortran, the language of Real Programmers.
Many packages have successfully used FP as a layer over those calculations, but that doesn't invalidate the original point you're replying to. Calling functions to add numbers is in fact that worst possible paradigm for performing the actual numerics.
