If you look into their tech blog post about the update, they use HTML5 as a fallback render for new stuff. They then create the native renderer for the widget and push it with the new update a few weeks/months later. So they can still A/B test as quickly as they want.
I don't think that they said that their fallback renderer was HTML5, but they did say that there were still areas of the app that still used that technology.
