this is likely because they aren't letters, they are arrows.
*** SPOILERS ***
And I can confirm that as of 11/12/19 that the only thing this game recognizes outside of selecting a server is the Konami Code. Anything else just triggers the key pressed sound while in the server list, and there is nothing inside the FPS part of the game that detects any kind of special code input.
You can not even load a terminal or anything to do the things the developer says to do to "load" the 100% save file, nor is there any code that looks for any kind of parsed codes (like a terminal *would* have to check for valid entries)