The title is incorrect. The Schengen Information System does not keep data for "all EU citizens", it only keeps such data for wanted persons, missing/stolen identity documents or objects (like cars, boats and so on). There is no centralized database for EU citizens photos or fingerprints (not even at national level in many countries).
What’s more interesting to me is in the Tweet: UK made illegals copies of hundreds of millions of people’s data (since you were mostly hung up on that, I qualified it better).
Are you seeing the interesting takeaway?