According to Lemmy documentation, if they don’t block a server, it means they are federating (aka talking using ActivityPub protocol) with that server.
Lemmy has three types of federation:
Allowlist: Explicitly list instances to connect to.
BlockList: Explicitly list instances to not connect to. Federation is open to all other instances.
Open: Federate with all potential instances.
Federation is enabled by default.
It means Meta could in theory talking right now with instances not blocking them as part of their testing.
I don’t know why they could not just block in the first place, then unblock/allow later if it makes sense.
Andreas