It sheds some light on the issue indeed but since a lot of that is in russian, I cannot decipher it. I don't see any command prefix there. From reading the command java files in your second link, I'm guessing the commands are defined as case strings there? Like set_hp, set_mp, spawn_item and such.
Edit:
None of the commands works with any prefix. Neither !, nor ´ or ` work. My account's accesslevel is set to 100 and the isGM variable is set to 1 as well.
Is there anything I'm missing? Special chat channel for commands, ingame toggle to enable them? When entering in /say chat, they come out as plain chat text only.