Mastodon doesn't fill your timeline with content from other instances, by default. If 100 million Facebook users federated with Mastodon overnight, 0 organic Meta-related posts would make it into my timeline. Only content reposted from Meta by people I follow would show up.

Defederated or not, Meta will have pretty much zero bearing on the instances outside their bubble.

All of this is true, but none of it contradicts what I said. Meta needs Federation to flaunt the bigwig researchers and developers from other instances. They will eventually build their own stockpile over time, but they wouldn't start with ActivityPub support if their intention was to eventually remove it. If Meta leaves the Fediverse, it goes back to how it is today. That's not a nightmare scenario for most people who are already using Mastodon, and it would put the leverage back into the hands of the users who could coerce Meta users to switch instances for more lax federation.

The ActivityPub ecosystem stands to lose nothing either way. Meta is walking a fine line with the remainder of the community that trusts them, but stands to integrate into a pretty nice system if they pull things off.