Blood donation and blood letting have other effects and confounding factors. You're supposed to be healthy to donate blood regularly, so right there you have an explanation for the observed longevity correlate. But it would also have other effects like reducing iron: people argue many men have too much iron later in life, while other donators like women may have too little, so that's mucking up your net effect.

Filtering blood, or diluting with saline, avoids these issues and helps get at the question of inflammatory & aging factors vs youthful factors.