When you see a warplane overhead in Iraq, and its backing Kurdish Peshmerga forces in a battle against ISIS, you automatically think of the United States. They are, after all, the one with hundreds of planes in the area doing that.
That Iran is involved in the ISIS war is hardly news, but the use of a warplane in a traditional US role is a major story, as it is all-but-impossible that Iran would be doing so without direct coordination with the US.
The US is desperate to the point of paranoia to say they “control the airspace” in Iraq, and having other nations’ warplanes just flying around willy nilly would make no sense, and would almost certainly make those planes a target. Iran would not be sending warplanes into Iraqi airspace in mid-US war, and in the vicinity of several US warplanes, without the US having confirmed that it was okay with them.
The US continues to deny any coordination with Iran and Syria on the ISIS war, but this denial seems to be primarily a diplomatic ploy at this point, and has no bearing on the policy.