"Being the product" refers to recording user behavior and processing it for gains. Displaying non-personalized ads (which are trivial to completely avoid in Vivaldi) is not that.
That's not really what "being the product" its 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.
You have a different definition of "being the product" than I do. A business that takes money from advertisers in exchange for your attention is selling your attention. You are the product is shorthand for your attention is the product, short of slavery.