The concept as I understand it is that Threads has the sheer volume of content to completely drown out the existing Fedi content if it fully opens the floodgates. If that occurs and say 90% of content becomes Threads and then they start making Threads only extensions to Activity Pub, servers will have to start patching those in and the Activity Pub project is defacto owned by Meta.
People also have issues with the Meta content moderation and the population on Threads, but as you noted that's fixable on an individual and community level. The existential threat to the future of the Fediverse is why servers should defederate. Meta can't and shouldn't be trusted with any amount of power over this community project.