I'm not sure if this [1] is still relevant, but it appears that Vivaldi makes money by promoting search engines and bookmarks to their users via their closed source, secret, Chromium fork.
If my usage of their Chromium clone is being used to sell search engines/website bookmarks, then I am indeed the product.
There does also seem to be a VPN option on their site that I'm assuming I can pay for, which seems it could be an actually buyable product rather than selling my usage of their browser.
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.
I’m really curious what gave you this impression. Vivaldi doesn’t hide its business model, yet you were so confident!