I’m not sure what you’re saying here—it seems to be entirely unrelated to declarative Shadow DOM. Can you explain more?
I think what they're saying is since the article says this "Shadow DOM provides a way to scope CSS styles to a specific DOM subtree and isolate that subtree from the rest of the document." if we could just make CSS namespace aware maybe we don't need the Shadow DOM after all.
But even then, what do you mean by “namespace”? Like, if we’re talking XML namespaces, CSS has that. But I don’t think that’s what’s meant. But I haven’t the foggiest idea what is meant (hence the request for clarification).