Fable 5 is constrained Mythos, which came out before April

Public disclosure of Mythos was April 7 and leaked happened in March, but it's been heavily delayed for well-known reasons.

That said, as the frontier moves, "months old" becomes more and more useful. Opus-tier models are being used to write serious software, so we're going to start seeing open models pick up a lot more usage imo.

Companies had Claude Mythos access in April. Low chance this is on that level.