Not the author, but I assume that to make it work with 32 bits integer coordinates, some operation (like multiplications) need extension to 64 bits. If we want full hardware support on 64 bits CPUs, that's the limit.
32-bit triangulation therefore requires 128-bit multiplication, in some rare degenerate cases.
In this repo the incircle is here: https://github.com/morishuz/delaunay32/blob/141d979b18e296ac...