Your existing collection of downloadable eBooks will be available to Kindle customers. As you add new eBooks to your collection, those titles will also be available in Kindle format for lending to Kindle and Kindle reading apps. Your library will not need to purchase any additional units to have Kindle compatibility. This will work for your existing copies and units.
Emphasis added by me. And I'll repeat it once more:
Your existing collection...
Currently, public libraries' existing collections consist overwhelmingly of EPUB and PDF files. If those existing files will now work with Kindle, with no additional purchase necessary, the only possible solution is that Kindle will support both EPUB and PDF.
And that is news indeed.