Turkey being concerned about that border is not believeable. Does Turkey do anything to secure it when arms and soldiers for ISIS cross both ways at will?
Turkiye, while not an ally of ISIS, hopes to benefit from ISIS and its Caliphate. The Turks believe the Caliphate belongs in Istanbul but are constrained from proclaiming it themselves. If the ISIS folks can get their Caliphate established and accepted generally then Turkey will undertake to move it and the new Caliph to Istanbul not without opposition from the Arabs.