A reboot doesn't cause you to lose data on your ephemeral storage. I think you mean "stopping" an instance and then "starting" it again (on another host), not rebooting it.
You are correct, that is what I meant, but I've taken to calling it 'rebooting' because all the intro-to-EC2 stuff I've seen listed their "how to reboot the instance" instructions as stopping and starting it, so I figured it's the appropriate vernacular, however incorrect it might otherwise be.