While what the Financial Times has done with polyfills is pretty neat, it at least does not document any of the behavior mentioned in this article.
In fact, it seems very clear that they are making use of App Cache for their website, which the technology mentioned in this article effectively replaces.
Polyfills emulate service workers as they were not actually implemented in any browser. So they have to fall back on App Cache. But now they can seamlessly use Chrome's native service workers while still relying on polyfills for other browsers.
Emulating service worker with App Cache sounds... very hard. Do you have more info on this specifically? I've got an app that's using app cache now and I'd like to gracefully transition it to using service workers as they become available.