The writing surrounding aetheric density doesn't feel very consistent. The Ancients are established as having their extremely long lifespans, creation magic, and proportionally immense size on account of it, but despite having over 50% of their density, people on the Source are interchangeable from those on the shards. For a while, I thought the explanation might just be that rejoinings only increase the overall quantity of aether on the source and don't actually make the existing people more "complete", but then there was that line in 5.1 about how exceptionally aetherically dense the Scions are compared to the denizens of the First. But then... They're still able to use dynamis, regardless? And then you have that comment from Yoshi-P about how reborn Ancients will probably just have some extra special powers but otherwise be normal, and it's like. What was making them immortal, then, exactly?

It feels like the writers were going for more of a rough vibe off-the-cuff than something meant to be mechanically analyzed.