I tried you example Flyer and it did not work. reason is easy, as you stated yourself, the command /mxc load has to be done from the Bot itself.
If you make a command like you did, it might well be posted in the room, but the owner of the command is the user who triggered the command and not the bot.
Metis cannot respond to the messages posted in the room by it's own scripts other than using out type="self" (which is not posted in the room) and in the case of control commands (/mxc ) by using out type="control" . Maybe you should check your files to see if the proper working script is present and making you think that script is working.
I'm not telling the help file is complete, but out type="usercmd" does not exist in the official metis releases.