There are about 18.446 quintillion more 64-bit integers than 32-bit integers.

True, but there are as many 64-bit integers as pairs of 32-bit integers.

Therefore the fact that relatively few 64-bit numbers are products of 32-bit integers means that a lot of pairs of 32-bit integers give by multiplication the same product.

I think they meant to write "There are about 4 billion TIMES more 64 bit integers than 32 bit integers".