Facebook is completely driven by ad revenue. There is nothing stopping them from sending out ads as posts/comments with artificially inflated scores, which would ensure that their ads end up on the "all" page of federated servers.
Threads already has more users than all Lemmy instances combined. Even if their algorithms don't apply to the rest of the fediverse directly, they can still completely dictate what the "all" page will look like for all instances by simply controlling what their own users see and vote on.
In general, Facebook has shown countless times that they don't have their users best interests in mind. They view users as something to exploit for revenue.