Hi Christopher,
Ideally, epubs need to be optimized for devices because they each have somewhat different support for various EPUB3 functions. For FXL ebooks, the devices also have different aspect ratios, so some designers who want to use up as much of the screen real estate as possible will create one FXL for iOS and a diff one for Android or Kindle. (Yes, Kindles can take FXL that haven’t been created with Comic Book Creator or similar.)
You don’t need to optimize multiple ebooks for one reseller like Apple, but if you’re going to sell the same reflowable EPUB on the iBookstore and the Kindle store, then yes, ideally you’d optimize for either platform if possible.
But Christopher, you are right that most middleman distributors like Ingram Spark only accept one EPUB file and distribute it to every ebook reseller. So it’s only those designers or publishers who work directly with Kobo, Apple, Amazon etc. who have the option to upload diff files depending on the recipient.
Hope that helps!
AM