There is satellite data that proves the plane must have been within a certain broad circular arc on the earth's surface. ISIS territory does not overlap even the northern part of the arc, much less the far southern areas believed (based on secondary evidence) to be MH370's most likely final resting place.
How much does that estimated range expand if the plane made a stealth refueling at Diego Garcia? (Rhetorical/hypothetical question)
If that satellite info is so reliable, why it can not pin point where the plane crashed? Why no debris was ever found?
The plane could have landed in any part of muzzieland under control of jihadists. That includes ISIS (Iraq & Syria), Yemen, Afghanistan, Waziristan province in Pakistan, etc.