Under the hood, yes, but Mythos had more relaxed safeguards and was/is only available to a subset of approved customers under Project Glasswing, similar to the situation with GPT-5.6 now.

Fable was available to me as a normal person using Claude.ai

Mythos never was and I don’t think that’s changing.

Mythos doesn't have the strict safeguards of Fable and is only accessible by a very small number of pre-approved companies.