If it updated while in Airplane Mode/when the WiFi was turned off...
Then my first assumption is "it wasn't really off, you were mistaken".
My second assumption would be "The update was downloaded at some point in the past and it just restarted". I'm not sure of the exact steps for an OTA update, since I don't usually wait for or allow them, but I don't think the framework runs them instantaneously, and I am not sure how long it might wait.
If it was instantaneous, that would randomly restart while you are reading, something Amazon doesn't want.
And I will contribute my agreement that "Airplane Mode" is not a lie.