It is a matter of faith, quite literally. But it’s just good to be aware that many other cultures don’t require a papal seal to acknowledge martyrdom. It’s been somewhat co-opted by the Taliban in headlines, but there are tons of Hindu, Sikh, Buddhist and other martyrs over time.
Tl;dr: these victims were innocent, and died because of their homeland and their faith. That’s why they are being called martyrs. It’s an apt descriptor.