The point of "super" coaches is to add the extra 2-5% to a player's game in terms of strategy, mentality, and so forth, to get them over the line at the biggest tournaments.
Whilst Zverev's second serve is how it is right now he's only winning big tournaments if the seas part and he's allowed to stroll through to the trophy with absolutely zero pressure and if that happens it'll be an insult to all the players who failed to win slams because of the big 3 blocking the way.
Maybe Zverev thinks getting a shorter guy in who could never rely on his serve for cheap points might help him sort out the mentality on the second serve... or maybe Ferrer's actually a treasure trove of technical insight... either way, he's going to need a lot of patience to deal with the Diva and his entourage if past super coach experiments are anything to go by.