I think we need more lore on the topic, everything is just speculation until proven true. There might be a chance that the fallen have stumbled on the pyramid ship on the moon, went to house of devils in hopes of achieving god hood because there are Siva on the moon from the reprised Shadow Thief strike, Taniks being brought back to life. Fallen could have been fascinated by the technology and with Siva with them they could want to bring up their tech to the pyramids tech. I’m just rambling, could be something but most likely it’s nothing