Rendering/layout engine is an admittedly sloppy term referring to the whole stack, except for the JavaScript engine.
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.