@jhowel...OK I'll try and be more specific and say that you can no longer upload an EPUB 3 file
as a fixed layout or print replica ebook on Amazon. For all fixed layout ebooks on Amazon, you have no choice but to use Kindle Create.
And if you don't believe what I say, then why not try creating and uploading a fixed layout EPUB 3 ebook to Amazon KDP for yourself? Then you would know for sure whether that method works, one way or another.
Also, when I did a search for "EPUB 3" on Kindle Jumpstart(Amazon Help), I only received two results and there was no mention of specifically using EPUB 3 format for print replica(aka fixed layout). You can see it for yourself here:
https://kdp.amazon.com/en_US/help?query=%22EPUB+3%22
And since I can find no Kindle Jumpstart advisory that specifically says that you
should use EPUB 3 format for all your fixed layout ebooks, then it would seem that using Kindle Create is the only way to create a print replica ebook on Amazon.