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How do we know this? As far as I know, Apple has been silent on the subject. I am a not saying that Apple won't use a proprietary DRM method on its ebooks. I am just don't think we know for sure yet. The article you linked to has a lot of IFs in it. Seems like it contains more speculation than fact.There are apps out that will read nearly any eBook format. However, the "built in iBook" app may not read the eBooks you are getting from there.
Also, the Apple Proprietary eBook format will be incompatible with other devices as well.