Chromium based Edge is definitely much more than the rendering engine. Do they use their own networking, process management, Javascript engine, extension system? It is probably still 80-90% chromium.
I think my point is still valid, that Microsoft hasn't given up on their browser, just most of the technology underlying it - which their users don't care about. The fact that Edge is a pretty decent epub reader (for now... https://www.thurrott.com/cloud/web-browsers/microsoft-edge/2...) isn't necessarily tied to the fact that it's EdgeHTML based. If they wanted to, Microsoft could implement the same feature set on top of Chromium.