Maybe not lie per se but they might have multiple data stores, one for the "social media" side of things and an archive for machine learning, one for targeted ads, etc. So they could legitimately say "we delete your data [from the perspective of social media]" while still having an archive for their other business units.

It wouldn't be the first time Facebook (nor any other business that profits from people's data) have been deceptive while treading a careful line between honesty and lies.

edit: I should add that if they were keeping archived records in a separate business unit, there is the possibility that they keep the original or even all edits. So scrambling your data might not do much aside confusing those who have you in their feed.