In Menes's letter, aside from the details of the battle, the one other thing that he greatly emphasized on was the capture of foreign troops, which also went on to explain how Perseus was able to gather double the estimated force.
In that letter, the general highlighted the finding of many dead bodies bearing telltale signs of armor designs identical to Thesos as well as the capture and confessions of many such men, who all claimed to be from various city states, and were sent here by their rulers to help Perseus defend the homeland of all Thesians.
All those men spoke with very distinct dialects and had unique gestures and clothing that gave irrefutable evidence of their identity.
And as Alexander read this, he found a loophole in this thinking, a possibility that he had not considered.
It had not even entered his mind that wars were not necessarily a two man's game, in fact, it rarely ever was.