Frontier model release cycles generally take around 6-8 months anyway. OpenAI and xAI (or however you spell it, branding almost as bad as X/itter) were probably working on their next generation of models already, and Anthropic just beat them 2 months to this release.
You also say "near-concurrent release of the same jump" - but 2 months isn't "near-concurrent", it's a full quarter of the normal release cycle.
I don't think that the other explanations you gave are implausible, though - for both human circulation and distillation, you can apply those during a training and development run (with reduced effectiveness). Reasonable to imagine those as bumping them up another few points to bring competitors from "a little below Fable" to "around Fable".