I agree that ArcGIS Enterprise Functionality Matrix doesn't do a good job of explaining ArcGIS Enterprise Kubernetes functionality. There are statements about ArcGIS Enterprise Kubernetes, but they are more along the lines of "see this" or "see that" for information rather than clearly stating how Kubernetes functionality relates.
That said, it seems like a simple question being asked, but it is hard to answer without knowing more of what you are really after. For example, since ArcGIS Enterprise on Kubernetes can federate with regular ArcGIS Server deployments, one needs to know whether someone's requirement is to only deploy in Kubernetes or whether a hybrid approach is allowed. If hybrid is allowed, than a lot more functionality and features are possible.
The other complication is that the underlying architecture of deploying ArcGIS Enterprise on Windows or Linux vs Kubernetes is fundamentally different. Esri may make the REST APIs match, and the browser apps look and function the same, but under the hood they are very differently structured. Because of this, Esri will sometime use terms like "equivalence" instead of "parity," and they focus more on describing outcomes and workflows than features and functionality when talking about the two types of architecture.
Overall, I agree with the sentiment that Esri does not do a great job helping customers understand the difference. Also, I really don't care for blogs as being "official" documentation, but the "Summary of Supported Functionality" in the What's New in ArcGIS Enterprise 12.0 on Kubernetes blog is about as authoritative as I have seen.
