The full code is

   static int leap_year(const struct cal_control *ctl, int32_t year)
   {
    if (year <= ctl->reform_year)
     return !(year % 4);

    return ( !(year % 4) && (year % 100) ) || !(year % 400);
   }

Where reform_year is the year the Gregorian calendar was adopted in the specific context specified (defaults to 1752 which is the year it was adopted by GB and therefore also the US).

So it does account for Julian dates.