There's a difference between Google "knowing" you sent a message and third parties being able to verify that you sent a specific message.
Whether Google stores your message when you turn on "Off-the-Record" is of course something we can never know. But if your contact says you said "X", and you say you never said "X", Google could still prove you said "X" at some point. That does not fit the idea of "Off-the-Record".
Whether Google stores your message when you turn on "Off-the-Record" is of course something we can never know. But if your contact says you said "X", and you say you never said "X", Google could still prove you said "X" at some point. That does not fit the idea of "Off-the-Record".