Because Claude doesn't allow third party harnesses on their subscriptions I doubt the majority of signals you're getting are actually that significant on pure model quality.
I suspect you're right on Sol not outperforming Fable; but i've not used Fable that much.
---
But, fwiw, in my custom harness between Sol & Opus 4.8 - then Sol wins by a ridiculous margin as Opus keeps claiming slightly wrong things with certainty much more.
You could say Sol is faster and cheaper and that's true. Outperforms Fable? Impossible to believe without hard evidence.