Fake. 9 hours have 32400 seconds. That means that he would have needed to collect about 8 diamonds per second. I don't think that that works out. Also for such a tunnel you would need about 80 million tnt try to duplicate that in 600k seconds with a duper made up by 10 tnts. And that doesn't even include the acceleration tnt.
Get three ghasts into a deep slate cave. Bonemeal moss in the cave while dodging the ghasts so they blow up the moss and uncover the ores that don't turn to moss. Use bonemeal in offhand and switch between hoe to get extra moss and pickaxe for ore. Night vision helps.
