No, admins might think of defederation as a way to avoid interaction with larger instances, but in the case of the bridge it was mostly regular users crying “I don’t my content going in a place that I do not control”, with “lack of opt-in” and “this violates GDPR” being the main reasons cited to be against it.
With Threads is the same thing. The whole thing with users asking their admins to block threads is not because they were worried about Threads pushing too much to the smaller instances, but to block Threads from mining data from the Fediverse to their profit.