|I'm with Kieran on this question. The first thing that strikes me about this accident & suggests what lessons might be important in avoiding a similar, very probably worse outcome in future, is that the belay is exposed to rockfall. The description on the mountain project thread refers to it being in a corner/gully or something like that. One of the hangers had already damaged by rockfall when they arrived. It doesn't seem like a great place to establish a belay & even though someone drilled there, the fact that another belay station had been built on the arÍte/edge of the gully really seems the more important thing here in terms of the key take away lesson from the incident. It is a dramatic illustration that if they had anchored using a sliding-x without limiting knots the cutting of the sling at that point would have lead to complete anchor failure whereas the tied-off sling featured redundancy that kept the belayers anchored, but that redundancy could be achieved multiple ways and none of these questions about how you connect to the anchor seem, particularly in this case, anywhere near as pertinent, especially in terms of how people are likely to be killed climbing, as the question of where you position that anchor/yourself.
Unlucky that it sounds like they had spontaneous rockfall, but, once it is in motion, a really lucky outcome for these guys, thankfully.