You probably need to interpret this as saying "the system is written in such a way that the application developer needs to take extra steps to secure their appl

You probably need to interpret this as saying "the system is written in such a way that the application developer needs to take extra steps to secure their applications, compared to how it could've been written". For example, privilege elevation in Linux is a very complex and complicated mechanism. The famed "sudo" command has a very long history of bugs. To the point that I remember RHEL peddling their own version ("ksudo" if memory serves... I cannot find any mentions of this, it was some 15 years ago). It's very hard to get things right. Especially, to prove that you've done things right, if you are after security. The system interface is designed "for comfort" rather than "for security". Many things could've been a lot more secure, but very tedious on application programmer (imagine SeLinux, but in hyperactive mode... that's every sysadmin's nightmare). In practice, with a lot of community effort, tools like "sudo" eventually reached a point where they are mostly reliable for what they are expected to do. So, to face practical threats, today, the system may be OK, but this isn't the testimony to its design, rather it is a consequence of how much effort was spent plugging the holes.