but have failed to read the specifications themselves.
What the specs say doesn’t matter if reality behaves a different way. Fact is that Beehaw blocked Lemmy.world but not the other way around and therefore Lemmy.world users can read everything from Beehaw. Lemmy.world blocking Threads would thereby be at best just a symbolic gesture and at worst actively driving people away towards Threads because that’s where they can access all the content. If an e-mail provider blocked all mails from @gmail.com, most of its users would jump ship towards a provider that doesn’t do that and perhaps even drive them towards GMail.