I mean, we can say "electromagnetic wave" instead of a "photon". It doesn't really change much in this case. I don't quite understand their diagram and their p

I mean, we can say "electromagnetic wave" instead of a "photon". It doesn't really change much in this case. I don't quite understand their diagram and their point. Are they looking at a region inside the collapsing shell of material? Then there's no contradiction here, the observer won't see anything until the reach the singularity (where the space-time stops being locally flat). Even if we consider an observer in the center of the shell, they'll only encounter the singularity after they get hit by the infalling matter.