Select ALL objects directly below an object

Is there a way to simply select all object directly below an objects?

I found this script here but it doesn't seem to work.  Any ideas?

//DESCRIPTION:Select Below
// A Jongware Script
if (app.documents.length && app.selection.length)
{
     ypos = app.selection[0].visibleBounds[3];
     for (obj=1; obj<app.selection.length; obj++)
          if (app.selection[obj].visibleBounds[3] < ypos)
               ypos = app.selection[obj].visibleBounds[3];
     for (obj=0; obj<app.activeDocument.pageItems.length; obj++)
          try {
          if (!app.activeDocument.pageItems[obj].hidden &&
               !app.activeDocument.pageItems[obj].locked && 
               app.activeDocument.pageItems[obj].visibleBounds[1] <= ypos)
               app.activeDocument.pageItems[obj].selected = true;
          } catch (_) { }
}

Here's an example file. I'm trying to select eveything below the yellow polygon.

This would be very useful in another project with many small objects bunch together below many larger objects. I want to group the small objects that are bellow the larger object together without having to manually select each objects (there are thousands of objects so a script would be ideal).