We investigate the issue and in the current version of the control pressing the escape keyboard key closes the DropDown portion of the control and it overrides the exit from cell editing functionality. We logged this as a feature requests in our Public Issue Tracker System. When it is implemented it will enable the possibility of overriding the functionality of keystrokes in the AutoCompleteBox control in order to achieve the desired behavior. You can vote for it and track its status here
I updated your Telerik points for bringing this to our attention.
All the best,
the Telerik team