You’ve completely missed the point. It’s not that Facebook (and by extension, their users) will connect to Mastodon, it’s that they will take over Mastodon, seizing all control for themselves, and coopting the existing userbase.
Right now it’s a separate product. Just like people know that Twitter is not Mastodon, Threads isn’t either. If you want to reach Twitter users, you get a Twitter account. If you want to reach Mastodon users, you get a Mastodon account. Facebook is planning to market themselves as the best way to enter the Mastodon ecosystem. Before long, they will be the absolute dominant server. Then they will have control, because defederation is a weapon they can wield and not vice-versa.
This is not theoretical, either. Google did the EXACT same thing back with Google Talk and the XMPP protocol. And we know how Facebook operates, so we know that this will eventually happen. The only way to stop it is before it starts - Facebook users need to be unhappy (at Facebook) that they can’t reach Mastodon users, so that defederation remains their own problem.
(Separately, I agree with you that Lemmy needs to become more accessible to the common user. But simply handing it all over to someone as awful as Zuck is not the way)