Wow, TIL. So you're saying you can get the audiobook legit through Audible and use a downloaded EPUB and it will still work if the ASIN is in the metadata? This is definitely worth a blog post or something at least.
Normally, books sent via USB can't use Whispersync to sync reading position between different devices. OP is saying that if you embed ASIN into the book (can just be a random ASIN) and sideload the same books to multiple device, they will sync, too.