Is not instances that don't sign the fedipact, as there are instances that haven't signed it, but are still blocking Meta-owned instances.
It's instances with admins that actively chose to federate with Meta that will get defederated, why? because even if a server defederates from Meta owned servers, if another server they federate with is federating with Meta, Meta still has access to data and information coming from the first defederating server.
My mastodon account is in a instance that signed the pact. This server already limits Mastodon.Social, but as the federation is not suspended, but rather limited, a user from my server, myself included, can interact with people on Mastodon.Social if we follow each other/I follow them or we are in the same group.
This mean that if my acquaintance on mastodon.social interacts with a post of mine, even if people on Threads cannot directly see my post, they still can see and interact with the comment made by that mastodon.social account. That way Meta gets data from all the people and servers that interacted with my posts.
So, the only way to really stop Meta from gaining data and information about and from users and servers defederating from them is to also defederate from servers that are openly supporting federating with Meta.