It doesn't distinguish, the non-digit character index is hardcoded into the bitmask (note the missing f in the second hex literal):

  part1 = _pext_u64(part1, 0x0f0f0f0f0f0f0f0f);
  part2 = _pext_u64(part2, 0x0f000f0f0f0f0f0f);

You are right that if you were to use a space in an unexpected position, the code wouldn't work.