It's two-way. It prevents interactivity between the instances, meaning that Mastodon doesn't get flooded with Threads users and Threads doesn't get access to Mastodon content.
Preventing both of those things is a win for the fediverse, because it preserves its identity and purpose rather than just being 10% of a network controlled mostly by Meta.
Allowing both of these things to happen is a win for Meta, because their users overwhelm the fediverse and they get free content until it no longer exists.
We don't lose anything by staying away from Meta, unless you like really love Facebook and want that to be what the fediverse is reduced to. Unchecked growth isn't a win, it's cancer.