Well the main idea behind Mesos is to create an aggregation layer that spans server capacity in local and remote datacenters. This aggregation does not tightly bind the servers into a shared memory system, but rather treats them like a giant resource pool onto which applications can be deployed. It is a bit like the inverse of server virtualization really.
Mesos runs quite nicely on CoreOS. Mesos doesn't replace the native Linux on each of the boxes in the datacenter. The Linux on each box provides the execution environment.
CoreOS is still restricted to one machine. It's about installing one operating system, which will run on multiple machines. With CoreOS you have X number of machines, running CoreOS, and Y number of applications running on top, with the CoreOS installations coordinating.
I think the point of the article is to have one operating system, running multiple physical boxes. You could then in turn have something like containers running on top of that OS.
