The nature of computer system penetration (hacking) is that it takes a great deal of time and patience. The attacker will put a lot of effort into learning everything they can about the system and then more time in probing possible vulnerabilities.

Linux and Unix systems in general have a better underlying security model than Windows (e.g., the way root/administrator vs. user is handled). Unix architectures also had years of students attacking them (back before this was a serious crime). However, if those of us who are Linux fans are honest we know that the reason we don't have to worry as much about Linux attacks is that hackers target Windows because it is more pervasive.

The Greenhills operating system has never been exposed to a large group of people who are willing to spend a lot of time penetrating it. The idea that you can just label a system as secure seems questionable. You always get attacked via means that you didn't expect. What they're really saying is that the system implements a security model that they believe to be secure. But B1 bombers are not placed on the Internet protecting large amounts of money, so they are unlikely to attract hackers.