Remember that temperature does not indicate energy. A graphene layer is a tiny amount of mass per surface area, so a absorbing single (small-c) calorie of energy would be enough to raise a patch of graphene to outrageous temperatures- but, you'd still only be able to get that single calorie back out, which wouldn't do much to a liter of seawater or whatever.

For that matter, it doesn't seem like they specify under what conditions this heating is taking place. Fire a high-powered laser at something and you might see that behavior in a variety of materials.