You'd think it would take a reasonably intelligent person no more than 10 minutes to work out the possible combinations of letters and be able to read the message.
It seems like that to me too but there must have been more to it than that. Those people were very intelligent.
Anybody could eventually decipher it but this wasnt a tool for storing intelligence long term like we do today. It was a tool for battlefield communication between command elements. If the enemy wanted to read an intercept or false flag a greek unit during combat then the enemy general would need to know that today we are using 8 sided 1/2” dowels and not the 5 sided 3/4” dowel.