The ?help “command” is not technically a command, and it is not seen as a command by Dyno. Instead, it is a response. This is why you won’t see it on command lists, and it is also why you won’t be able to disable or modify the command settings in any way.
The response is meant to help people, and it will respond via direct messages (unless a valid command name is provided). You cannot disable it in your server.
The only way you can stop it from responding is if you remove the permissions for Dyno to speak in any channels you don’t want it to respond in, however please note that taking these permissions will also mean Dyno cannot respond in the channel to any other command, and a lot of features will no longer work for you.