Basically this.
1. To my knowledge, this isn't the first time being afflicted with fel or a fel-tainted wound was a death sentence, regardless of healers (And no "My character gets stabbed by demons 1462 times a night" is not a counter argument)
2."Light in his eyes fades away" trope usually a literary/theatrical way of guess what? Confirming a character has died.
3. He literally starts turning to ash on the throne (Also potentially answering why they burned the body, rather than mummifying it)
4. We definitely see an arm on the pyre, presumably, the rest of his body is under the fire, unless we're getting into arguing that Vol'jin, healing powers already taxed beyond their limit, cuts off his own arm, ties it to a Vol'jin shaped pile of straw, sets it on fire and rides off into the sunset as Zul'jin 2.0
He's dead. Now, for trolls, death doesn't always mean gone (See: Gara'jal the Spirit Binder, Jin'do, etc.) He could very well being using his untimely demise as a way to be recruiting the Loa to our fight with Sargeras and the Legion.