Depends on their methodology. Sure, a huge proportion of those are users who haven't heard of uBO, but we're forgetting a lot of caveats:
- Electron exists and lots of apps are built on top of it and identify as "Chrome". Judging by the numbers most have been weeded out, but some edge cases do visit more sites so they end up in the count.
- A lot of workplaces mandate the browser, which is often Chrome. This also gets counted.
- A not insignificant amount of Firefox users change their useragent to Chrome.
All of these skew the numbers towards Chrome. Some Chrome users use a different adblocker which lowers the uBO statistic.
Lemmy isn't a single website like reddit.com is. It's rather a collection of decentralised servers ("instances") offering the same service (one very similar to reddit). It's often compared to e-mail - just as Gmail users can talk to Outlook users, lemmy.world users can post and comment on lemmy.ml from their home instance.
What this does is it removes the centralised aspects of Reddit - if a community has powertripping mods one can make an alternate community (like on Reddit). But this goes a step above - powertripping server admins can be reigned in by simply switching instances.