No thanks, just throwing a tarball a someone else face is not making anything "open-source".

That's source available, not open source.

You can view the source, but you can't contribute to it, create forks, etc.

Source available is one of the prerequisites for software to be considered open source, but not the only one.

They have a page [1] asking the question "Is Vivaldi Open Source?" which does not answer in the affirmative. Instead, it says "for all practical purposes the Vivaldi source code is available for audit", which is quite a different thing.

[1] https://help.vivaldi.com/article/is-vivaldi-open-source/