Before we can try to draw any conclusion we need more data of course. It shouldn't be too dificult to confirm if the other gliders had flarm. If only one glider has flarm it is as good as no flarm. The pilot in the pics mentioned that the other glider came out of nowhere. This should not happen with working flarms.
BTW I believe PowerFlarm has big advantage over the classic flarm who only gives you few seconds of collision alert while PowerFlarm gives you a better display providing situational awareness so you have plenty of warning that someone is nearby and how many gliders are nearby in the gaggle.

Ramy