as much of a dickhead as he is, ulfric has a surprising sense of honour and respect for the greybeards. even if he used it to incapacitate the guy, i doubt he would have used the thu’um to actually kill him.
i don’t really trust elisif or stentor for that matter. elisif probably doesn’t remember all the details right because she was so distraught to see it unfold. sybille stentor is also a literal vampire who’s deceiving an entire hold so you tell me if she’s really a good source of informationEDIT: torryg says ulfric only “unleashed that terrible voice” or whatever when you meet him in sovngarde but be never says it’s what killed him. the rest of his dialogue is just about how he misses elisif and then it’s just the generic “the world-eater waits in the mist” dialogue