Well, I was trying to keep the explanation simple for Sal. If the adaptive radiation of the two possum types followed the aftermath of the Cretaceous-Paleogene extinction event 65m years ago, then they are indeed related (as marsupials) in the sense of being in the same "Infraclass", but not related as they are of a different taxonomic "Order". It's a matter of how far back you want to go to say any two species are "related"; go back further still and us humans are also "related" to the possums as we're all mammals (but that's not very informative here).