When I said they referred to it as low-level, I wasn't just making shit up. From Stéphane Graber's (LXC lead) blog:

"Instead, LXD is our opportunity to start fresh. We’re keeping LXC as the great low level container manager that it is. And build LXD on top of it, using LXC’s API to do all the low level work. That achieves the best of both worlds, we keep our low level container manager with its API and bindings but skip using its tools and templates, instead replacing those by the new experience that LXD provides."

> A container is simply a file system in a folder that gets booted and works on any underlying Linux filesystem so I am not sure about how conceptually a 'container format' fits there?

Well, then maybe you should read TFA? It's all about the creation of a standard 'container format'.