Parts of it are. Vivaldi wants to retain its brand and doesn’t want people making forks and potentially tarnishing its reputation. And given how unpopular it is, they can’t really afford that to happen. I personally have no issues with Vivaldi wanting to keep things that way and I don’t mind it not being completely FOSS. Given how absolutely amazing the browser is and how customizable and feature-packed it is, it’s absolutely irresistible for me not to use it.

Here's a blog post from Vivaldi about it not being completely FOSS and their reasoning

They've also got a great privacy policy so I'm not concerned with privacy either.

Fun fact: Vivaldi is the go to browser for car makers such as Lamborghini, Mercedes, Audi and other car manufacturers https://vivaldi.com/android/automotive/