Spring Preview: Harbaugh self reflects on 2017 coaching
Throwing versus man and throwing versus zone are quite different, especially if you are getting a real heavy diet of one or the other in practice.
Training the QBs eyes is very important: if you are throwing against man every day, you may not get the practice you need reading the safeties, their rotation, and "the box". Versus man, you can get away with a bigger windup and delivery, since the defender on your target receiver is looking at his man and not the QB. There's also the problem of the QB dropping his eyes to look at the rush, rather than keeping the eyes downfield. Versus zone, the ability to use the eyes to look off defenders and not allow other defenders to see the QB and break on the ball becomes vital. It also requires timing- you can't just lock on a receiver and wait another couple of steps to come open like you do in man- by then the receiver will have moved into an area of other threats, all of whom have their eyes on the QB.
Actually, coaching WRs is something often left to graduate assistants or less experienced coaches, depending on the configuration of the coaching staff, so that is not that surprising. (WR and RB are positions that are more dependent on outright talent as opposed to a position like LB or OL).
But especially with a lot of young, inexperienced talent at WR who are struggling, you'd expect to see a bit of help in coaching them, such as from the OC if he is not coaching a position coach, or the HC himself.