If you look your SVG file, you will see that each shape is enclosed in a group. So you need to go into each of these group to get the shape inside.

1. PShape seats;
2. PShape[] seat = new PShape[150];
3. 
   
4. void setup()
5. {
6. size(800, 600);
7. 
   
8. seats = loadShape("kamer_plekken02.svg");
9. 
   
10. for(int i = 0; i < seat.length; i++){
11.   seat[i] = seats.getChild(i);
12. }
13. smooth();
14. noStroke();
15. 
    
16. for(int i = 0; i < seat.length; i++){
17.   seat[i].disableStyle();
18.   fill(random(255), random(255), random(255));
19.   shape(seat[i], 0, 0);
20.  
21.   PShape[] s = seat[i].getChildren();
22.   println("C " + s[0].getVertexCount()); // all 0
23.   println("CC " + s[0].getVertexCodeCount()); // all 0
24. //  PShape test = seat[i].getParent();
25. //  println(test.getVertexCount()); // all 0
26. }
27. }
28. 
    
29. void draw() {} // Needed to get mouse events
    
30. 
    
31. void mouseMoved()
32. {
33.   for (PShape sh : seat)
34.   {
35.     PShape[] shs = sh.getChildren();
36.     PShape s = shs[0];
37.    
38.     if (s.contains(mouseX, mouseY))
39.     {
40.       fill(#FF0000);
41.       shape(s, 0, 0);
42.       break; // No need to check further
    
43.     }
44.   }
45. }

thank you so much.