I would normally suggest they're making far too many assumptions without some hyper detailed per-user analytics and those would be really hard to associate the Viewer with the Buyer of goods. Especially on a Quarter by Quarter basis. They might have some visibility into it, but I'd be really surprised if they can get that type of resolution, outside of of Viewership to SC conversion.
Nijisanji isn't that well run. If we take their likely over-assumption as fact, it means their entire Margin is basically in JP buyers of merch. Which likely means they aren't selling merch that a Western Audience wants to buy. Cover might eventually clear 5 million USD a year in plushies alone with how well those have been selling, while at the same time switching far more to a sponsorship/IP type revenue model.
The one thing that I really think people miss on the regular is just how well run Cover is. Yagoo deserves a massive amount of credit in this regard. Cover has its issues, but they've fought against the brutal "fishbowl" mentality that pervades Japanese companies. Someone that knows Moonrune idioms a lot better could find the correct one, but a Japanese friend explained the philosophical approach. It kind of translates out to "fishbowl", but we'd call it "big fish in a small pond" while not quite representing what happens. It's "anything outside of Japan doesn't really exist".
Nijisanji has that Anime Production Company issue where they don't really know how to sell outside of Japan and have failed miserably. Cover knows how to sell outside of Japan, they're just pretty screwed by the still super high air freight rates. They badly need a distribution hub out of the States.
So, if everyone is Hololive was going to cover the song, might as well have nearly everyone cover the song? haha