Conceptually, I agree. But at youtube scale, I suspect they aren't comparing a corpus of "asserted copyrights" versus "all videos". It's a corpus of "all videos" (straight up) with some preference for asserted copyrights.
I expect that even toggling off Fortnite's "licensed music" setting can still run into audio copyright claims against other Fortnite videos which have the same audio.
Yeah, it's common for Influencers with a big subscriber base to shove all their videos into Content ID, so other smaller channels get their videos claimed because they happened to play the same video game (and the audio matched as a result).
AFAIK in that case it's at least not malicious, it's just the content ID system completely failing to handle this category of content, where a big chunk of the visuals and/or audio are guaranteed to match with other videos because it's a recording of interactive media.
I expect that even toggling off Fortnite's "licensed music" setting can still run into audio copyright claims against other Fortnite videos which have the same audio.