Script to select same stroke color of path group ~~Adobe Illustrator

I had just opened page 3, it comes as I showed with no clipping mask

if you open the 3 pages, each page is a group. The group you're interested in is the top group or page 3.

all you have to do is target the clipping mask, which is also a group, 

try this

var doc = app.activeDocument;
var artb = doc.artboards;
var artLayer = doc.layers[0];

var topGroup = artLayer.groupItems[0]; // *** added
var clipGroup = topGroup.groupItems[0]; // *** added

var s = clipGroup.pathItems; // *** edited

var whiteRectangles = [];

// collect white rectangles
for (var i = 0; i < s.length; i++) {
  var pathItem = s[i];
  
  if (pathItem.strokeColor.cyan == 0 && pathItem.strokeColor.magenta == 0 && pathItem.strokeColor.yellow == 0 && pathItem.strokeColor.black == 0) {
    whiteRectangles.push(pathItem);
  } else {}
}   

for (var a=0; a<whiteRectangles.length; a++) {
    artb.add(whiteRectangles[a].geometricBounds);
    whiteRectangles[a].remove();
}