It's a bit hard to explain, and I really don't mean that in a patronising way. I guess I meant it in the way that you can't flag a non-executable file with permissions that could make it executable.
OS400/IBM i/whatever they call it now has a very robust object model and security system. A file can only be flagged "executable" when the system is happy to call it so, e.g. it's been successfully compiled. You can't just add ".exe" onto the end of a (for example) specially crafted mixed text+binary file in the hope of causing a problem, you can't patch binaries directly, and you have zero access to the binaries of the operating system, except of course via patches and upgrades sent from IBM.
My statement was a bit simplistic, sorry 'boot that.