For the Rust context, “almost all first-party components are written in Rust”: https://github.com/bottlerocket-os/bottlerocket.
Wasn’t CoreOS the same thing?
On a high level, yes. The README has some links to details.
CoreOS was originally based on Gentoo, although I think they just used Gentoo/Portage to bootstrap their binary build system because there was no emerge. They had a package manager but I don’t recall what it was, except that it was minimal. The last shop I was at that used CoreOS was using fleetctl and flannel. We migrated off of it onto DC/OS before Redhat got bought out by IBM.
By contrast, this looks more like a BSD where the entire base is one big install and gets patches as such.