Confusingly, that page only provides the changes to the Google Chromium source that allows their UI to run. (I'm not sure it would be easy to discern this without already knowing the source is not fully open.)

https://help.vivaldi.com/desktop/privacy/is-vivaldi-open-sou...

Tarballs every 2 months, and we know these don't give you the Vivaldi browser as they supply it.

I don't trust them one bit. There was that telemetry analysis that showed Vivaldi as a very noisy browser.