Unfortunately, the MarkerClusterer reference is a bit scant. After browsing through the source code, it looks like you need to call the getMarkers method of the cluster object passed in (contrary to what I had previously suggested, which was to call the method on the markerClusterer).

For example, using the tutorial you've linked to:

google.maps.event.addListener(markerClusterer, 'clusterclick', function(cluster) {

    var content = '';



    // Convert lat/long from cluster object to a usable MVCObject

    var info = new google.maps.MVCObject;

    info.set('position', cluster.center_);



    //----

    //Get markers

    var markers = cluster.getMarkers();



    var titles = ;

    //Get all the titles

    for(var i = 0; i < markers.length; i++) {

        titles += markers[i].getTitle() + n;

    }

    //----





    var infowindow = new google.maps.InfoWindow();

    infowindow.close();

    infowindow.setContent(titles); //set infowindow content to titles

    infowindow.open(map, info);



});

EDIT: Updated in response to question edit.