Oh dear. Given the audience for this article I'd assume that readers know that this configuration is not suitable for production... but then again it would probably be a good idea for the article to state that!
Restricting /dev/kvm these days doesn't make much sense. The interface is designed to be safe for any user. The fact that we started as a character device and not syscalls is just a historical decision.
Since only the Firecracker user needs read/write access, it would be trivial to limit that to just the Firecracker user or group.
Step one, here is a guide that effectively removes all protections on the host system.
Win.