This is not at all what I study, but it seems possible to me that sponges have evolved a tolerance for low oxygen conditions in the billion or so years since they first arose. It's a common mistake to think that because something is morphologically very similar to an ancient form (a so called "living fossil"), that it hasn't undergone any evolution. As long as it has been reproducing and mutating, better adapted versions have always had differential success, hence they've evolved, even if the shape hasn't changed.
I thought the point of the study was that these sponges live in an oxygen-rich environment, so there is no pressure for them to develop this trait in the modern world. It doesn't prove it's a primitive trait, but it at least makes it unlikely that it's a recent adaptation.
Is the "oxygen-rich environment" universally true, though? It's not as though oxygen saturation in the ocean is perfectly homogeneous. There are plenty of naturally occurring areas of hypoxia, and one could easily imagine an adaptation which allowed better survival in or near these "dead zones" could be quite useful, at least to localized populations. Is there some obvious reason that dramatically reduces the likelihood that one or more pockets of sponges might have capitalized on such a mutation more recently?
It's not a completely novel idea, but it it an interesting find which may lead to more interesting conclusions.
I tuned out when they started dating the emergence of a group of animals by working backwards based on a guess of how long they would have needed to achieve a certain level of genetic diversity.
That's interesting as conjecture, and I'll take it as such.
recently read about early life conditions on wiki [1], says pretty much the same thing. also relevant [2]
That's not about early life. It's about early animals, that only came to be much, much later than the former, when oxygen was already plenty at the atmosphere.