I don't get what would defederating with Facebook-federated instances gives you, though.
Site A hosts communities that serve vulnerable people. They see Meta as a threat to those vulnerable communities, as they are not well moderated, and have no issues with hate speech and harassment, so they defederate.
Site B federates with both Site A and Meta. They act as a pass-through for content from Site A to reach Threads.
Bad actors on Threads see content from vulnerable people on Site A and engage with it. People from Site A cannot see the bad actors on Threads doing this, but people on Site B do, and bad actors there get alerted to an opportunity to be proper shit stains. Now, vulnerable people on Site A get targeted by this induced harassment coming from Site B.
What does Site A do?
They defederate from Site B.
The question is just about whether they wait until the harm has been done or not.