Fediverse memes

In the scenario you suggested, a user who has blocked a harasser should no longer be aware of continued harassment by the harasser. Thus while the mods may have to step in, there is no particular urgency required. Also, a determined harasser will just alt-account no matter what the admins do, regardless of the blocking model used.

BlueSky just passed 21 millions users.

BlueSky isn't really comparable, since they have a user-user interaction model as compared to Reddit / Lemmy which have a community-based interaction model. In a sense every BS user is an admin for their own community.

there would be more than 4 or 5 people who would call out about misinformation.

Agreed. However, good faith users by nature tend to stick to their accounts instead of moving around (excepting the current churn b/c lemmy is new). Regardless of how many people would call out disinformation, it's ultimately not too difficult to block them all. It can even be easily automated since downvotes are public, meaning you could do this not just to vocal users fighting disinformation but anybody who even disagrees with you in the first place. An echo chamber could literally be created that's invisible to everyone but server admins.

Can’t we use here the same argument other people use about Lemmy being a public forum, and thus the posts being public for everyone except the blocked accounts?

We could, but again, good faith users tend not to be browsing while logged out. They have little reason to do so, while bad faith users have every reason to.