He's not ok, his problem has nothing to do with pinion angle.
Again, the mechanic is correct, and knows what he's talking about. You do not want the driveshaft to be in a perfect straight line from the trans to the rearend. This has nothing to do with pinion angle, that's a separate measurement. The driveshaft wants to have a downward angled pitch to the rearend, this makes the needle bearings in the u-joints spin. If the driveshaft is straight, there is little load on the u-joint bearings, and the joints could just turn without spinning the bearings. This will cause what's called "brinelling", where the bearings staying in one place puts indentations into the bearing surface of the u-joint. The result will be a u-joint chatter, better described as a bad vibration.