Earlier quoted context omitted.
It's not stupid, it's true - a lot of the UI is written in HTML/CSS/Javascript.
Like Atom? So it is not unique. We can say this statement for a lot of software products being Chromium more special and from a third party.
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).