Granted, but from a consumers perspective both lead to the same result.

I still remember the announcement -
"[...] for all new products Opera will use WebKit as its rendering engine and V8 as its JavaScript engine. It’s built using the open-source Chromium browser as one of its components. Of course, a browser is much more than just a renderer and a JS engine, so this is primarily an “under the hood” change. Consumers will initially notice better site compatibility, especially with mobile-facing sites [...]"

They came up with a completely different browser. And they didn't call it Operium or smth. (Yeah I'm still a bit mad about announcement vs. what they shipped. Been a long time user before - even paid to remove that little ad when it was still a thing.)

Opera used to have this feature way before but they decided to drop it for some reason when they changed their rendering engine to chrome. It might be interesting to know that the creator of Vivaldi used to work for Opera for quite some time. That's likely the reason why they are quite similar to some extend.
I'd love to get this feature back but there been already a couple of requests for this without luck so far.