They'll be using chromium to some extend, that doesn't mean it's going to be a chromium browser that's exactly like Chrome, Yandex, Torch, SRWare and what have you. That is what you seem to imply.
As you probably know "some stuff on top of that" is rather broad in Opera's case, assuming they keep current functionality. If they have business sense they'll also want to keep their 300 million users, I doubt they'll do that by killing their product and making an Opera branded Chromium instead.
It states that for the user, it's going to be the same browser. It also states Opera extensions will keep working, so at the very least we know they'll be making something not as chrome as the other chromium browsers.
Yes, exactly, running Opera extensions on top of Chromium is precisely the kind of stuff Opera should add on top. Not that hard to do too, and does not require a fork.
It won't be exactly like Chrome, I never said it would - it will add stuff to it and remove some stuff. And I do think that can be a viable product that keeps many of their users.
I agree Opera might be more different than Chrome than SRWare for example. But it will still be 99% chromium in the core engine. To make significant changes in there would mean forking chromium and not being able to easily update from upstream - which would be silly.