I just tested this on my laptop (which I keep on the stable channel). For some reason, I needed to reload the page to trigger the update check.
After the update to Chrome 28, I still had Flash 11.7. The blog post says that Flash 11.8.800.97 (the latest version) is rolling out via the component updater. I don't think there's any UI to trigger a component update, but it should happen naturally by itself.
Here are the current versions of Flash: http://www.adobe.com/software/flash/about/