I'm not really sure of what you mean by false positives.

After my understanding of the stacking technique as used in other contexts in astrophysics and without going into too much details :

The problem they were facing is that the signal to noise of the images of the filaments was too low to say that they had detected anything in any individual images. However by stacking (adding) images they were able to detect it because the signal grows roughly with N (N being the number of images) and the noise grows with sqrt(N). So by stacking enough images you'll get the signal to noise necessary to say you've detected sth.