What does autonomous mean in this context?

That it can recover by itself. It's a bit more than just 'auto-restart' (as mentioned in the article) since it's an entire vm that is restarting versus a process and it's not the same as restarting a linux vm.

so this is LXC based or is it something else? trying to figure this out as well.

Nope, no linux. https://github.com/nanovms/nanos is a new kernel designed to run a single (linux) application as a virtual machine.