I believe this is a vulnerability that occurs when copying data from dom0, which is a more common task. Generally the Qubes model recommends copying data from more trusted VMs to less trusted VMs, and dom0 still runs some system-wide processes in many default configurations.

For example when you take a screenshot with xfce4-screenshooter, the file is saved to dom0, and you have to use qvm-copy-to-vm to move it to a (less trusted) qube to do something with it. That's the most frequent use case, at least for me.