(making an answer out of the comments)
In order to indicate I do not understand any AT commands at all (aka I am not a modem) the correct response to any received AT commands should be silence.
In order to indicate I do not understand this particular AT command the correct response should be ERROR
.
Anything between will trigger implementation defined behaviour of the entity sending AT commands. Some will possibly give up right away while modem-manager apparently is set on retrying sending the command until it gets a "proper" response.