there's more to "the general public" besides 2 instances. beehaw defederated from .world and sijw because the mod tooling to handle a huge influx of people isn't ready, and it still isn't ready. (and the rest of their defederations are an off the shelf mastodon blocklist import which all instances should do imo and a few explicitly unmoderated instances. oh and porn i think)

beehaw federates just fine with the instance i'm on, for example.

if they wanted to defederate completely, lemmy does support allowlist federation, and i'm pretty sure their admins know about it.