100% guaranteed, cast-iron, watertight security is impossible - but VDIs could potentially prevent whole classes of attack. It's about increasing the barrier against realistic threat models.
I completely agree - many attack vectors are more difficult, and some are impossible. However, saying "you can never get the data out of the cloud except via screenshot" is, first of all, untrue, as other means exist, and second, preventing access to data is not the most important security aspect of such a system.
