What they did is tried to prove that the conventional fuzzer won't find crashes. Where they found counterexamples, they fixed the bug and tried again.
In essence, a fuzzer takes random samples out of a space of possible inputs that could crash. They proved that nothing in the space of possible inputs could crash.
This is a strictly more powerful result than a fuzzer could give you.
What they did is tried to prove that the conventional fuzzer won't find crashes. Where they found counterexamples, they fixed the bug and tried again.
In essence, a fuzzer takes random samples out of a space of possible inputs that could crash. They proved that nothing in the space of possible inputs could crash.
This is a strictly more powerful result than a fuzzer could give you.