So...about that Shadowbringers trailer.

[Seraphor]

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.

[Yuyuka3]

Specifically Gamer Escape's Lorecast 9. (Most questions I've seen about lore in general that have "I saw the answer in an interview" seem to have that interview be Lorecasts 7 or 9, for some reason.)

Gamer Escape: Would a “Light-born” void differ from the “Dark-born” void we know?

Koji Fox: Actually, yes. There’s that that “blank, white perfection” line in the game. That’s accurate. The Dark void that we know is void of aether, but you still have the people and animals that were there and they’re aether-starved and trying to break through into our world and suck that aether back into their own. Whereas in a light void there’s NOTHING. It’s empty. Beyond that I can’t say anything.

So that was why people were surprised at the whole reveal of Sin Eaters, and now we have what look like Haukke Halicarnassus palette swaps as Sin Eaters.

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.

Except that line implies that the 13th is, indeed, a dark void. Koji explicitly states that they are very different and that things can exist in the dark void, but not the light void. Therefore, while the Sin Eaters will most likely wipe themselves out when the light void is complete, the voidsent can exist in their dark void.

I do agree with on the rest, though, and I actually think that is why voidsent can exist in their void... if that makes sense.