Whether or not that is true, Russia’s SU-24 was clearly a combat aircraft with its IFF turned off operating unlawfully across Turkish territory in defiance of many warnings to identify itself and reverse course away from Turkey. These same Russian aircraft have been violating standard rules of engagement by intimidating the Turkish air defenses by locking-on their weapons’ radar targeting systems ever since the previous incident in October. Turkey was entirely within its rights to shoot down the apparently hostile and unresponsive warplane.
If it was shot down over Turkish airspace, yes. But if it was shot after it had left Turkish airspace, and was merely a flyover, shooting it down was an illegal overreaction, so, no. That's like shooting a trespasser in the back after he fled your property without doing any damage. That appears as if it may have been the case.
I am sure ISIS totally agrees with your assessment.