Unique?
What a joke.
It just loads other websites inside an iframe. That's how all similar web-based apps and web-OSs work. Eg there is also servo with its HTML based UI; there is WebOS with its browser, ChromeOS with its browser; FirefoxOS; etc - totally non unique beside it has a nice UI inspired by the old Opera 6-12 UI.
I'm pretty sure that they mean Vivaldi is unique as a browser built entirely using modern web technologies.
PS: I'm not saying they're right, I haven't researched to find out if there are other HTML/JS based browsers (aka with browser chrome built in HTML/JS).