The BBC isn't hosting anyone else on their instance (nor can I think of a reason they would want to). As I understand it it's just a way for their activity to be visible on the fediverse. That should make their infra costs minimal; they don't have moderation of comments/replies; and federation/defederation wars will only affect the specific other instances which choose to defederate from the BBC. Your third point is valid but that's why this is an experiment.