Some people say that Meta makes Threads federate with Mastodon so it would eventually siphon its users (see #EEE).
That's one of the reasons several Mastodon instance admins decided to preventively block Threads (see #Fedipact).
I think that keeping some Mastodon users from interacting with Threads might on the contrary push them away from their instance and maybe go to Threads instead, what we want to avoid in the first place.