I think this is a bit hyperbolic. Fable will fall back to Opus.

> Unlike our interventions for cybersecurity, biology and chemistry, and distillation attempts, these safeguards will not be visible to the user. Fable 5 will not fall back to a different model. Instead, the safeguards will limit effectiveness through …

No it won’t fall back to Opus, they will purposely return dumbed down or tainted information with the goal of the end user not knowing the results have been impacted.

That’s a separate mechanism, and it will tell you so if it does (if the prompt is remated to cybersecurity, biology)