The article does not state at any point that the written test cases involved actual exploit code, and this is also very unlikely given what we know about Fable. Even if they did, it would not in any way be exposing the ability that originally raised concern wrt. Mythos Preview, viz. staging realistic cyber attacks that would be able to work around non-trivial defenses and chain vulnerabilities in a goal-directed way.

Opus can very much "fix the code". Quite possibly even Sonnet can. This is a big fat nothingburger and it's increasingly looking like the political restriction of Fable at least (not Mythos itself, of course) was arbitrary and based on the flimsiest pretext.