I hadn't heard of these other Cisco products until yesterday, but from what I read IOS XE and NX OS also run on Linux.

Anyhow, getting back to my original point about FreeBSD, there's a few sources that have linked IOS to FreeBSD. But after doing some digging of my own on this topic, I've come to the conclusion that this is one of those urban legends that's proliferated for whatever reason. So it would seem that you're right about IOS being home rolled. Interestingly though, they do have other products which are FreeBSD powered (eg AsyncOS http://www.cisco.com/c/en/us/products/security/email-securit...) but, as you said, not IOS