I have a theory, this is obviously based on speculation based on how Anthropic is treating Mythos and the whole media noise around it's dangers and who gets access to it.
My theory is that Anthropic are banking on being the top model when the race to IPO finally reaches the finish line, and to do that they need to have the top model but not let any competitors see it or derive from it to have a comparable model in the market.
Fable is their way of showing the public "the model does exist but in a mode that makes it harder/impossible for competitors to derive a comparable model from results.
That's definitely the case as model distillation is one of the explicit safety carveouts they mention. Though TBF, model distillation is also a big concern for general safety as distillation could allow you to have the model without the other guardrails. It's sort of a master key to the model.
The irony of "we train on all of humanity's collective output, but god forbid anyone trains on ours" is still incredible