https://nanos.org which is for the kernel itself, is indeed not Linux. It's a custom kernel we wrote from the ground up.
The basic premise is to run one and only one application as a vm - typically in a public cloud environment but can run on things like rpi4 as well. Doing so allows us to run things like go/rust workloads 2x fast on google cloud and 3x fast on aws. Takes ~20 seconds to deploy.
One of our principal kernel engineers discusses some some of it's salient points in this video: