Is integer addition FIP?

[chtenb]

I'm reading the paper about the FIP feature of koka https://www.microsoft.com/en-us/research/uploads/prod/2023/07/fip.pdf
My understanding is that the fip keyword requires a function to do no allocations or deallocations, but owned memory can (must?) be reused. All arguments to a function are owned if the ref count is 1, unless they are marked with ^, which means they are borrowed.

I'm looking at the int implementation at https://github.com/koka-lang/koka/blob/dev/lib/std/core/int.kk and notice that addition is marked as fip.

inline fip extern int-add : (int,int) -> int
  c  "kk_integer_add"
  cs inline "(#1 + #2)"
  js "$std_core_types._int_add"

// Add two integers.
pub fip fun (+)(x : int, y : int ) : int
  int-add(x,y)

However, to my understanding ints are arbitrary precision and therefore can be stored in heap-allocated memory. So then worst case I think adding two integers would require one allocation and two deallocations. Why is this function still marked fip?