Earlier quoted context omitted.
> An xlsx file is just a bunch of XML files in a zip. A bunch of XML files with opaque formats that MS constantly makes changes to to make its competitors have to keep chasing the format.
Even if it were true it wouldn't matter a whit to the production side of the format, which is what "produce CSVs using fprintf" is: excel can consume them all.
Libreoffice handles normal UTF-8 encoded, quoted value CSV files fine. Excel not so much.