Almost all existing real world software is full of holes and security flaws. Mythos is better than humans at uncovering many of them; especially because its time is a lot cheaper than that of the top tier human experts (and even of mid-and low-tier human experts).

Especially when these systems are written in notoriously unreliably languages like C.

I don't think Mythos is especially good at writing systems that are free of security problems. Essentially the only way we know is by proving your software correct.

In principle, you can even prove C correct, but in practice you'll want to write your system from the ground up to be proven correct instead of adding that property after the fact; and for that you'll most likely also want to pick a language that supports this better.

See https://en.wikipedia.org/wiki/SeL4 for a noteworthy example.