That is a valid criticism of their setup process. My guess would be that "No Blocking" is set as the default option to ensure that the average user clicking through the setup process does not encounter difficulties accessing cerain parts of the web and mistakenly attribute it to Vivaldi being an inferior browser. Like all browsers, their target market will be people moving from the market leader, which is currently Chrome. As is well known by now, Chrome does not provide users with these protections by default and so many of its users do not know or care about them and just accept the experience as normal. Vivaldi and others therefore base their default installation options on what a Chrome user would expect, as opposed to what is objectively the best setup for privacy. If a user does care about their privacy, they are almost certainly going to select another option during setup.
I think a fairer and more relevant way to compare these browsers would be based on their optimal, in-house GUI setup options, without going into things like Firefox's about:config or extension stores. To me this is a more realistic way to present information to a user who is concerned about their privacy and looking for a new browser. The assumption that someone concerned with their privacy would just blindly install a browser and never enter the settings or make any adjustments is a pretty silly one. Vivaldi would still not be the best, but the tests would better reflect its ability to offer privacy to its users.