Just in case that one isn't working for you I also made a similar background extension you can download it here and uploaded it into your instance....

https://github.com/mike-st/parallax-background-switcher

That a side it could be your Version of iOS if it is an older version, make sure you phone is up to date or the extension as been updated and perhaps that feature was changed by mistake. Which means the demo is using an older version of that extension that works.