Hook defender would be the CB who is covering the WR on a (button) hook route (sometimes called a curl). If the defender is on the right side of the WR, the QB should throw it to the left side of the WR - ie away from the defender.
a Dig route is like a deep in. Say 12-15 yards. It’s a dangerous throw over the middle so, a qb might throw the ball low, so his WR has to slide to catch it, and this not get smashed.
The quote was talking about the awareness and accuracy of Darnold reaching another level.