I'm not really sure what you mean when you say "I'm not buying your quick dismissal": I never dismissed anything nor made out either that there is or isn't a large difference between the engines. I just corrected a statement that is definitely factually incorrect (and unfortunately believed by all too many).

On the actual differences between them, Chrome may be "almost identical" to Chromium, but unless you're working for Google, or have very comprehensively disassembled it, the differences is not easy to measure. What could be tiny in binary size could still be a massive change in behaviour, particularly in terms of things like telemetry.

Beyond that, they are of course not equivalent, they're just analogous. Finding a few tiny chinks in the comparison is not the same as invalidating it though. The primary analog is the approach to interop, and the resultant behaviour of the development community. Participation in standards bodies is an improvement over the IE6 days, but if you take a deeper look at Blink development, you quickly realise it's a pretty insignificant one in practice. Google controls the HTML5 spec. (the editor is a Google employee and the WHATWG is not a democratic body like the W3C), and outside of that the Blink team basically just implement what they want and specify later, rather than the reverse. The response of the development community: supporting the monopoly by ignoring interop - is pretty self-evident from links like this one.