It’s not much of an assumption. Most of the first reports stated it. The evidence at the scene supports it. The video of his body first being pulled from the burned out building shows smoke around his mouth and nostrils, blue lips and smoke smudged clothing. He appeared lifeless.
Those first reports also state that an “American security team” or “American commandos”were flown in thru the Libyan dessert to resue the 30 left in the consulate.
That may look that way, but really, we know nothing about what happened there.