I'm not sure what you think this means.
Airlines sell a booking for carriage between A and B. The ticket you are given is just the documentation of this booking.
Airlines do not sell seats. They may sell a seat assignment but this is different

When you cancel a booking your ticket becomes invalid for travel. The airlines revenue management system may or may not increase the available inventory and then offer that inventory for sale. Even if you cancel a booking and the airline sells another booking to someone else it does not mean your returned inventory was used..

It is very unlikely that anyone at Easyjet can tell what happened with your booking and its related inventory on this flight. Even detailed history searches on inventory and booking for he flight may not reveal if your booking was "resold"

Even if the flight was full and oversold it does not necessarily mean your booking was resold.




But even if it could proved 100% that your booking/inventory was resold you are not entitled to a refund because you had a non-refundable booking.
Since you cancelled on medical grounds you may be able to claim on your insurance, if you have insurance