I think what followed this quote made quite a good job at explaining what exactly he meant with this statement: That people tend to have a limited view of math as calculus only.
I was introduced to programming before abstract algebra, and when I took my first algebra class the comparison to Haskell was inevitable. Actually it turned out to be quite helpful: strongly-typed functional programming can be a good supplement to learning algebra.
In maths, you often write something like let x = ...
An imperative language statement like
x = x + 1;
isn't something you see in maths, that is not a valid equation.