It feels to me like Fable is just a slightly more advanced Opus 4.8 (or 4.6?) but with this 'adversarial' self-challenging/checking of work and a more compute to really hunt down edge cases or to spin up many sub agents using lesser models. That's what makes it feel like a big jump, but I think the results wouldn't be so different if you manually challenged 4.6 with enough iterations of logs, screenshots, and follow up questions.