Which is funny, because they say that popular operating systems are not designed with security in mind. They give this as a reason to start from the ground up. I am from the camp saying that currently sole kernel does not an OS make. With this in mind network equipment OSes are certainly not popular ones. Nevertheless there are probably lots of less popular OSes with less popular kernels that are designed with security in mind. Some of them are probably good tries at such a goal. They could be used and extended and it would be probably better securitywise, because of years of bug fixes, that no new OS can have.