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?"
Amazon's not going to openly advertise that this deprecation is also the line in the sand where "non-DRM ePub just works", but that's what has happened.
Of course one of the sadder problems with the ePub ecosystem is that it uses the same file extension for DRM contained and non-DRM contained ePubs. At a glance it isn't easy to tell if an ePub is not DRMed. Amazon does not support any of the existing ePub DRM schemes. Their own KFX DRM is very unique and proprietary and doesn't play nice with ePub DRM "standards". You can't load DRMed ePubs over USB, those don't work. Sometimes that gives an impression still that "Amazon does not support ePubs natively", but that's the nature of DRM and how much DRM hurts the entire ebook industry in every direction.
I also tried renaming the ePub to a .kfx and it still did not work.
Your anecdote also seems to be the only instance of it working natively. Keep in mind Calibre will autoconvert for you.
PDF were just not meant to be viewed on the old one, but the 11th gen handles them surprisingly well.
Also, maybe the publisher of that book in 2015 wants to upgrade to new ebook features for that book in 2026, for instance they want to add the physical book's original illustrations now that Kindle finally supports more illustrations. Does Amazon have to keep both of the 2015 and 2026 versions of the book depending on which device the user wants to use? How confused is the user when some of their devices have lovely illustrations and others don't? Should the user be able to choose to read the 2015 version of the file even on devices that support the 2026 version because they hate the book's illustrations and find them distracting?
(That gets into a larger discussion that Amazon has always preferred updating books in place on kindles with later editions as they are published, which archivists hate especially because the kindle doesn't have a great "edition version number" to rely on to track for when Amazon has delivered an update to a file, but which often consumers prefer because typos slowly disappear and books subtly become better than the last time you read them, presuming the Publisher isn't doing some drastic bait and switch and it focused only on "plussing" the book.)
KFX is the modern kindle format, AZW meanwhile is heavily PDF-based. KFX was designed ground-up by Amazon, supports every modern feature they could think of, and presumably couldn't be backported to 2013 and earlier Kindles; AZW meanwhile was basically a wrapper around a subset of PDF. KFX is a complete redo, notable enough it's what "Enhanced Typesetting" on every Kindle product page means, not a small DRM upgrade.
By doing this, all authors will soon receive guarantees that they will have the full KFX feature set when designing eBooks, and won't break AZW by accident. Trying to point this out though to the "it's about DRM" or "it's about obsolescence" crowd will get you downvoted to oblivion before the truth is even considered (speaking from experience, -4 when I dared suggest legitimate reasons exist) and is a prime example of echo chambers and deeply ingrained bias on this forum.