He says kindle books - so he means the kindle ebook format - on other mobile devices - by which I assume he means mobile devices running kindle software, like the reader for the iphone.
Don't expect support for anything that might directly compete with the kindle (such as the Sony or other eInk devices). He's aiming for the people who won't splash out on a Kindle because they've just spent a similar amount on a netbook or an iPhone.
This isn't opening up the supply chain in any way, it's just extending it to people who are adverse to buying extra hardware to read books. These are the customers who wouldn't buy the Sony or Cybook either.