Here's one scenario.
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Facebook feeds its users content according to an algorithm.
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Facebook and lemmy users can interact with the same user content (liking, commenting).
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There are vastly more Facebook users than lemmy users.
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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