Better portrayal of Mozilla's response from this ars technica article:

The vulnerabilities identified by Mythos could have also been discovered either by automated “fuzzing” techniques or by having an “elite security researcher” reason their way through the browser’s complex source code, Holley writes. But using Mythos eliminated the need to “concentrate many months of costly human effort to find a single bug” in many cases, Holley added.

The key part there is fuzzing. These tools have existed for a while. (and many are free!!!) Mythos just does what most AI tools do: Take something that requires more understanding or effort, and condense it into a prompt. Instead of starting a fuzzing tool, configuring its scope and some parameters, then letting it go hog wild for a bit, you just tell the AI model with a prompt to perform similar functions. (while costing more money and taking more time due to inevitable overhead from running a whole LLM)

If anything, this points more towards Mozilla not using existing fuzzing tools to find flaws in their code because they were too lazy, not that Mythos is magic and superior to all else.