The differing magnitude estimates may have come other sources than the one you cited, however, the central point of my post is confirmed by your cite: at 2:59 PM HST the quake occurred. (t=0) At 3:07 (t+8 mins) they first became aware of the quake at all. At this time, islands nearest the epicenter have already been struck by the tsunami. 3:14 PM (t+14 mins) they issued a bulletin stating no threat to pacific nations. By this time, mainland Sumatra was already being hit. At 4:04 PM (t+1 hr 5 min) they revised quake from 8.0 to 8.5. By this time, Sumatra was already destroyed.
At approximately 5:30 p.m. (t+2hr 31 min) Internet newswire reports of casualties in Sri Lanka provided PTWC with the first indications of the existence of a destructive tsunami. As I posted, they did not have advance warning of the tsunami, and incidentally the first indications of the destruction came from across the Indian ocean.
You are right however that American authorities thought the quake was in the 8s, not the 7s, when the waves struck.