Your EC2 instance with instance-store storage when stopped can be launched on any other random host in the AZ when you power it back on. Since your rootdisk is an EBS volume attached across the network, so when you start your instance back up you're going to be launched likely somewhere else with an empty slot, and empty local-storage. This is why there is always a disclaimer that this local storage is ephemeral and don't count on it being around long-term.
I think the parent was agreeing with you. If the “local” SSDs _weren’t_ actually local, then presumably they wouldn’t need to be ephemeral since they could be connected over the network to whichever host your instance was launched on.
