That's not was certified software means.
You know hw software is issued "without warranty for any fitness or purpose, whether stated or implied" (or something like that)? Thats the key differentiator. There are OS's certified to that level. You know that line from Fight Club about when a recall is / isn't issued? that holds the same for OS develpers that provide a product warranted for fitness and purpose. These OS's do exist, for those situations that require it.
You see them in car computers, for example, and I'm confident the Falcon9 OS is certified to that level. It's not that it's bug free - it's that the developer has done a higher level of regression testing, and continues to evaluate the OS to identify problems that could (or did) arise, and with that agree to share in the risk. (Here, "risk" is defined as effects that happen to the company when the product causes a kick in the teeth... like killing a person.)
Certifying anything as 100% bug-free is a waste of time, since it's nearly impossible. TeX isn't even there, despite 40yr of offering bug bounties. Noone claims 100% bug-free, but there are OS vendors that accept a certain level of risk.
weylin