In Aoe 3 it was possible to chat with ALL players in-game or a specific player but now we can just speak to all players or just our team. It’s a little feature which would be really handy, especially with the “Diplomacy” option in Advanced settings mod.
Yes as you said it should be possible to chat with a specific player especially because of the diplomacy option. It is also interesting for all versus all matches.
By the way they should add the advanced game settings mod to the base game.