Nobody’s saying that, in terms of user bases, the Fediverse is comparable to Facebook or Instagram

Well, maybe I got the wrong impression, but I felt like the userbase of the fediverse was implied as the motivation for Meta federating.
And I wanted to put in a comparison, why I don't think that this is the case.

I don't see a reason why Meta should want Threads to federate, except for "well, whatever, doesn't hurt us to get those fractions of a percent". They'll probably have to use whitelists anyway, due to different legal situations on different instances. So at best they'll federate with some of the bigger instances.

Most of us have been on Facebook or Reddit and have given up on those bigger communities and adopted the Fediverse because it aligns with our values and privacy principles.

I'm sorry to tell you, but your privacy isn't exactly great here.
Every Thread, Comment and Upvote at least can be requested from any fediverse instance.
And do you know what, you don't even have to be a fediverse instance yourself to do that.
But I guess you knew that, so you're here because nobody tracks what you look at, which is great, and because you like Open Source.
That's not going to Change when Meta Federates.

Facebook does not. Its Fediverse platform will not suddenly be the opposite of what the company has been doing for more than a decade.

That's true.
But it will be two things, if I may steal the analogy of someone else in this thread:
first it will be a black hole ripping through the Fediverse.
I'd like that to do as little damage as possible.
I'd love it if mastodon continues to grow after Metas release, and doesn't collapse under server costs, Spam and other detrimental effects.
For that, preparing for the coming storm seems useful.

second it will be a huge amount of possible connections, of people.
I'd love to be able to toot a reply to some meta thread.
I mean, wouldn't it be nice if the fediverse would already know certain rules that meta may require to federate with them? And I mean sensible rules, like no/flagged porn, issues with piracy etc.
One could also talk about how Meta allows/blocks instances. A lot of legal trouble for Meta could probably be avoided if they only show posts from a whitelist of instances, but any user could post to their instance.
But how would they deal with non-whitelisted instances trying to pull Threads-Content?
Maybe they want to talk about how to deal with those "half-federating" situations, because this is not the current norm, and they may not actually get more bad press when a meeting could have prevented it.

For both of these effects I think communication with meta can only help.