That has little to do with robustness. Linux may very well be as robust or even more robust than any OS considered for those purposes, but the question isn't robustness, it's predictability.
I even go further and say that Linux is more robust than many of these other OSes, since it is exposed to a wider range of environments and hardware combinations of varying quality and whatever bugs they trigger. If you reduce Linux to the kind of footprint that, for instance, VxWorks or QNX have, I don't believe it would be any less reliable.
But Linux isn't predictable. Many safety-critical systems only rely on the system not crashing or misbehaving, but many others rely on real-time characteristics. Those Linux doesn't have.
I even go further and say that Linux is more robust than many of these other OSes, since it is exposed to a wider range of environments and hardware combinations of varying quality and whatever bugs they trigger. If you reduce Linux to the kind of footprint that, for instance, VxWorks or QNX have, I don't believe it would be any less reliable.
But Linux isn't predictable. Many safety-critical systems only rely on the system not crashing or misbehaving, but many others rely on real-time characteristics. Those Linux doesn't have.