Facebook feeds its users content according to an algorithm.
Facebook and lemmy users can interact with the same user content (liking, commenting).
There are vastly more Facebook users than lemmy users.
By dint of Facebook's greater number of users, lemmy users will see the most popular content that is fed algorithmically to Facebook users.
Conclusion: lemmy users are being fed content by the Facebook algorithm (in this still, thankfully hypothetical, scenario).
Like imagine Facebook promotes some viral post and it gets a thousands of upvotes. Any lemmy user on a federated instance, sorting by upvotes/hot/etc, is going to see that post.
That's the kind of top-down reach that is so alien to the fedi