Meta can easily harvest ActivityPub federated data without implementing ActivityPub federation in Threads.
The biggest concern is overwhelming moderators in no small part due to the lack of knowledge, planning, and experience one should expect of amateur administrators and moderators to have in dealing with moderation concerns when there's a sudden 3x (or higher) order of magnitude increase in federated accounts.
That and the inevitability of a huge increase in ads and sponsored and commercially driven content.