Personally I think that non-Orthodox Christians, who were not aware of their heresies, but died for Christ, truly are martyrs. It's more complicated with non-Orthodox Christians, who not necessarily were fighting with Orthodoxy, but they knew there is the Orthodox Church and died because of Christ.
I don't know how to explain incorruptible body of Roman Catholic martyr Andrew Bobola, who was killed by Cossacks. Maybe it's just because he was killed for Christ, and in this case it doesn't matter that the murders were Orthodox Christians, because they shouldn't have killed him?... Or maybe that's a work of demonic forces?... That's the most interesting case for me, as in Poalnd on the opposite, Orthodox site is st. martyr Athanasius of Brest