[rhymedawg]

So, I figured it out:

You must choose "Both," where Calibre gives you the choice of Old, New, or Both.

Simple as that. Uploaded to Amazon Kindle Direct Publishing (KDP) sans problem.

Moreover, footnotes appear on their own individual page with a nice "back" arrow for the user to get back to where they clicked the footnote.

And finally, book is reflowable so it can be read on Kindle devices.

My writing environment is Google Docs, which outputs files in docx format. So, the signal path is a lot simpler. Google Docs to Calibre to KDP.