My understanding is that the actual features needed to provide containers are part of the Linux kernel. What LXD provides is tooling and a daemon running as roo

My understanding is that the actual features needed to provide containers are part of the Linux kernel. What LXD provides is tooling and a daemon running as root that are used to manage containers. There's no reason why these can't be part of a snap, it doesn't introduce another layer of sandboxing for the actual containers. (That said, I would still prefer that they not be.)