The original AZW format was MOBI-based, not PDF-based. MOBI originally from a company called MobiPocket, which Amazon eventually acquired, was built to be an ePub competitor and like ePub was an HTML and JS-based solution, but in a somewhat different, proprietary DRM-friendlier container format. (ePub is "just" a ZIP file, with the DRM applied sometimes inside the container rather than outside it.)

MOBI stopped keeping up with ePub standards and standard features, in part because Amazon acquired MobiPocket. The KFX is just ePub with a new proprietary DRM container around the ZIP file that is ePub's container.

The 2013 boundary is also the "supports ePUB files directly without a conversion process" boundary in Amazon's kindle OS. It's not just useful to know for book file authors, but as a consumer it becomes useful for a quick "Can I buy a standards compliant DRM-free EPUBs such as from sites like DriveThruFiction and just send them to my Kindle with no other steps?"