"Lost control" in this case means their armed forces are now protecting that territory and any governance is done by them. They do, as you say, act more like a state in this. But of course many would prefer not to suggest that.
You should remember that the only difference between a terrorist organisation and a state is what people call them. Perception is everything, so people will still refer to them as a terrorist organisation. However, they could well meet every definition of a state, since they do control territory. But their control over these territories is not stable and things are changing so quickly that lines can't be drawn.
Even if they stopped expanding and managed to defend their territory for a hundred years, under the current ideology they espouse they still wouldn't have a diplomatic relationship with other countries. "Diplomacy" means that both sides are willing to offer something to each other. Islamic State is founded on the very simple idea that their version of Islam is superior to every other way of life and that they will impose it on everyone else. They want nothing less than global domination.
The fact that their tactics for dealing with other nations has been to try to scare them indicates what we're dealing with. They've already claimed responsibility for an attack in the US. They've essentially declared war on the world.
Remember that a state that declares war on another removes all diplomatic ties. So you could say that it's equivalent to a rogue state, but on a completely different level to any other in modern times. Even North Korea and Iran don't use every capability they have to try to expand. IS will, and everyone knows that.
I seriously doubt that there will ever be any communication that doesn't involve missiles and bullets.