The First isn't a 'Light Void' though is it, it's in the process of becoming one. Much like the Void on the thirteenth probably isn't a true aethervoid, because beings exist.
It's like, space is not a true vacuum, if there's things in it.

Voidsent are aether-starved, while Sineaters are aether-gluttons.
This means that Voidsent sustain themselves through their actions, they cling onto 'life' by seeking out aether.
Sineaters fulfilling their nature would instead keep gorging on aether until they reach a tipping point and that aether consumes everything, and there's nothing but pure light aether left.