Agreed, I don't believe it to be true at all, either.
Let's face it, the people who are that invested in reading that they'd consider a Kindle are likely to vastly prefer reading on a device purposefully built for it.
My Kindle Paperwhite is gorgeous. I also have an iPad 4, and a rMBP. As great as both those devices screens are, the Paperwhite wins, without a doubt, for reading. Even the iPad is too big, too heavy, to comfortably hold up or above my head for any period of time.
People so heavily invested in reading that they purchase many eBooks using Amazon's infrastructure are likely to have investigated the hardware designed for same. And whilst my preference for the hardware over an iDevice doesn't automatically mean everyone will find it the same, I find this claim highly tenuous.
