Interesting.
Of note, DL has a section of your online profile to set “Delta Sync On Demand Preferences”, described as:
Delta Sync On Demand Preferences
Manage your seatback screen personalization preferences. Opting out of personalization will not affect your ability to watch entertainment on Delta aircraft. Once saved, your selections will take effect the next time you connect to Delta Sync On Demand.
I have all opted in on DL, but never noticed anything personalized on IFE to date.