Meta is allowed to use the ActivityPub standard just as much as any other standard. This does not mean anyone who decides to use it must interact with others who use it. SMTP will block your mail if you aren't from a larger server, have the right signatures and even then. Servers block HTTP over VPNs often, and there are even rules about referencing content via other servers on HTTP (CORS). Just because a standard is open doesn't mean everything using that standard has to communicate with each other.

The beauty of this is that those running instances can't restrict access of other instances to the fediverse. If Meta does start using ActivityPub, every current instance can block it. Other entities could want to run an instance that federates with Meta who has the resources to do so. Currently the biggest issue is the vast difference in scale between current instances and Meta. But if other entities got into the fediverse that federated with Meta this would still be a decentralized system, just with larger nodes between them. All of this still allows those who run small instances to block these larger instances that are more mainstream and keep it the way they want it.