I'm not sure if this is true. Harold Abelson creates the distinction between Mathematics being the study of truth and Computing being the study of process. It seems to me that these really are different things and Mathematics isn't the "natural language" to discuss computations, but rather truth and patterns. But of course process (computing) can only happen within the boundaries of mathematical truths and patterns.
 https://www.youtube.com/watch?v=2Op3QLzMgSY the first few minutes
They still approach computation using mathematical reasoning methods. Note how they define car and cdr and how they approach problems in those videos.
I believe Abelson and Sussman use the kind of mathematical reasoning I am talking about in all their work. SICP being a prime example.