UrlFetchApp.fetchAll is definitely async.

testForSync.gs

/* The following code produces the following output in stackdriver log (newer logs higher):
Time for concurrent: 5.848 seconds. 
Time for sleeping 10: 5.003 seconds. 
Time for sleeping 2: 5.002 seconds. 
Time for sleeping 9: 5.004 seconds. 
Time for sleeping 8: 5.002 seconds. 
Time for sleeping 5: 5.003 seconds. 
Time for sleeping 3: 5.003 seconds. 
Time for sleeping 1: 5.003 seconds. 
Time for sleeping 7: 5.003 seconds. 
Time for sleeping 6: 5.003 seconds. 
Time for sleeping 4: 5.003 seconds. 
Note that the sequence is not predictable, and if it were sync then it should have taken 50 seconds to complete.
*/

function timeit (name, ctx, func /*, args */) {
  var now, then, args, ret;
  now = Date.now();
  args =  Array.prototype.slice.call(arguments, 3);
  var ret = func.apply(ctx, args);
  then = Date.now();
  console.log('Time for ' + name + ': ' + ((then - now) / 1000).toString() + ' seconds.');
  return ret;
}

function sleep(seconds) {
  timeit('sleeping', Utilities, Utilities.sleep, seconds * 1000);
}

// this code needs the Requests.gs modular library: https://github.com/classroomtechtools/modularLibraries.gs/blob/master/Requests/Requests.md
function concurrentProcessingTest() {
  var response = timeit(
    'concurrent',
    Import.Requests,
    Import.Requests.concurrentlyInDevMode,
    function (c) {
      c.add('sleep', '1');
      c.add('sleep', '2');
      c.add('sleep', '3');
      c.add('sleep', '4');
      c.add('sleep', '5');
      c.add('sleep', '6');
      c.add('sleep', '7');
      c.add('sleep', '8');
      c.add('sleep', '9');
      c.add('sleep', '10');
    }
  );
}