1. Technical. I don't think it's apples to apples. You don't need a single You don't need a single fediverse instance to handle 9 digit MAUs, and if you think of each subreddit as a different instance of a fedi-server then I think it would hold up, and I'm sure it could with some work.
2. Social: I don't think the strike/black out will succeed in any significant way unless the mods have more leverage. Threatening to go fedi, and starting to do it for some of the subreddits, will dramatically change the power dynamics. As a thought experiment, game out the current situation. Reddit needs to make money, and if they want to IPO (i am anti IPO but that's not my choice) then they will also be expected to grow exponentially. They can't do that with third party apps that don't show ads so they are stuck between a rock and a hard place. The community very much wants to have these communities, and everyone knows that. If they can stick to the strike it's mutual self-destruction but from reddits perspecgive given in is also self destruction. The CEO is right, the community likely won't be able to maintain it for all kinds of reasons. The main one being reddit can just make the subs public again if they want to. So what leverage do they have? None if reddit believes their only option is reddit.