Kaspy OS runs on a switch, and they're talking about popular operating systems, so in the same vein, OpenBSD wouldn't be a popular secure OS for e.g. routers?
But hey, I'd be really happy if they based it on seL4 and formally verified their security concepts. That would be a real game-changer. OTOH I'm really sceptical until they provide any relevant details.
To make a verifiably secure system for network infrastructure and IoT-devices, you need, at the very least, a provably correct IP stack. Want a nice web interface? Now you need to verify the HTTP server, too. Want to talk to other devices? You probably want a DNS resolver. And so forth...
Simply signing code and having the OS refuse to execute code without valid signatures is not going to be sufficient to convince a lot of people that it's a significant improvement security-wise.
(If, on the other hand, they make it open source and provide proofs of correctness for all these components, that would indeed be a significant step forward.)
But my point was that it still leaves a whole lot of attack surface. If you want a provably secure system, you basically need to verify much more than "just" the kernel.
On the upside, if one did so, it would have benefits beyond security.
http://www.lynx.com/lcs-lynx-certifiable-protocol-stack/
The minimum I've seen are a few drivers, bootloader, partitioning filesystem/storage, partitioning networking, and language runtime (esp Ada or today Rust). That's not a lot to build given what's already available in FOSS or embedded. More than just a separation kernel but not much more.