alt Hacker NewsThat's not really what "being the product" is commonly recognised as.
Vivaldi go out to their customers (that's not you) and say "We have 1000 suckers who have downloaded and use our closed source, Chromium cloned, browser. We can serve your website as a bookmark to them, or add you to their search engine list if you give us $x."
Since you are not their "customer" (that's the people paying them to appear in your settings) what are you? You are the "product", you are what Vivaldi is selling.
That's actually NOT what this phrase implies. It is really about targeted ads and the drive for more ads and more precise targeting based on user data. Look up attention economy.
Vivaldi is not that. The only place where it shows anything remotely like ads is its default start page with prepopulated links and that can be hidden by flipping a single setting. I haven't seen this page for at least a couple of months now o my Vivaldi instances.