"still the *same* render engine" is poor wording on my part. Of course they overhauled/reworked things, dropped support for ActiveX and a lot of backwards compatibility stuff. So it probably would be more accurate to say it's a fork of the old engine.

That edgeHTML starts at version 12.0 is a hint for being a continuation of that lineage - but there's also a few bugs/idiosyncrasies edgeHTML has inherited from Trident.

I remember that edgeHTML did spoof the user agent so browsers would not detect it as IE, and would not apply IE specific hacks and polyfills. And that would cause problems with some web apps, that could be fixed by re-enabling a select few hacks/polyfills.

It's been a long time ago, so I don't remember what specifically it was... sorry.