Interesting article. I'm not much of an OS guy, anyone can shed more light into the subject? Why the object oriented operating systems was integral to the OS's ability to withstand intrusion?
http://homes.cs.washington.edu/~levy/capabook/
PS - According to the Wikipedia article on System/38, IBM removed the capability-based addressing from the AS/400.
There's nothing inherent in OOP which is going to make it magically secure. It just seems like a spurious fact about OS/400 (and I say that as someone with very little knowledge of it, although I did use it as an 8 year old). Moreover, (maybe) there's no public record of infected instances, but that doesn't mean it didn't happen.