Lord Theony met Alexander in private to ask him to let his son Gnaeus lead the right flank of the army, and thus help the man obtain some much needed military accumulation.
"My lord, I have been training my son since childhood about warfare. He will certainly prove his mettle! All he needs is a chance."
The man pleaded on his progeny's behalf.
"Mmmm…" And looking at the hopeful, almost glassy pairs staring at him, Alexander put on a pretended thinking hum,
"Okay," before Alexander subsequently lightly nodded in agreement.
Since the flanks' jobs were so simple- just stand, defend, and shoot arrows, Alexander did not find a reason to be too fussed over who the commander was there.
Even if the man was not as competent as Lord Theony was making him out to be, unless someone was actively trying to sabotage the formation, Alexander figured he could put a monkey in charge and the men on the frontlines could still figure out the things on their own.