The real problem here is that it's not specified in any case. It could be that there were some material differences between the newly minted and ancient souls, or it could just be the maturity cycles meant it would take very long for their decimated population to recover. As Veloran notes, it is indeed referred to as a portion of this new life.

Either way, Venat herself does not speak ill of them and says they only do as they believe is best for the star. And I think that's the crux of the disagreement - the two groups had conflicting visions on how to best steward the star. Checking Zodiark's might and ceding the future to the 'newer souls' was their view on how to do that. Again the way they don't malign the Convocation suggests to me they're also viewing their proposed actions in the framework of sacrifice for the greater good of the star - they just disagreed that further sacrifices would achieve that. Their concerns therefore also appear to be pragmatic in nature. There may be more to the whole thing but based on what we currently know, that's how I read it.