It’s not, though. The way to boot a transient OS (just like a transient instance of a container on a machine/instance with a container runtime) is to give userspace read-only access to the image. It can be outright read-only or it can be an actual efficient overlay mechanism (qcow, overlayfs, device-mapper snapshot, etc). EBS, as the article notes, can’t actually do a read-only mount of a snapshot at all, and it’s very inefficient at instantiating a volume from a snapshot.
