I agree with him completely, why are people so he'll bent on fear mongering this? The worst possible outcome is that the fedi will... be exactly the same as it is now lol.
Meta doesn't care about the Fedi population, they're not even 1% of their userbase, and they know that the fedi crowd is one of the most anti people of their network.
There is no embrace-extend-extinguish. If Meta starts to change up the ActivityPub protocol and then make it proprietary, then networks like Lemmy and Mastodon will just stay on the original one. They can't force the fedi to follow, and they know this.
They can't inject ads or data scrape more then they already can. Your public info is already public, which can be assumed that it can and is being scraped. This exact comment is one of them. They can scrape your info the exact same as they can now.
This will introduce more people to the concept of the fedi and they'll be more willing to migrate to other platforms like Mastodon/Lemmy when they understand the concept better. This is only a good thing for the population, and we won't lose any to the new network as stated before.
So at that point, what is a single downside of this? You can even just instance block them if you still dont like it, so it won't even affect you then.