Title is misleading. They're not rejecting apps that code in support for Pebble. They're rejecting apps that say they support it in the App Store copy. This is nothing new; you've never been able to mention other platforms in your App Store copy.
You simply cannot use Apple's platform to advertise alternate products. Just like you can't mention Android or Chromecast in your App Store copy. Put it in the app or on your website.