My understanding is that there was OS version years back that added EPUB native support, but Amazon never properly announced it, it just quietly happened. It's

My understanding is that there was OS version years back that added EPUB native support, but Amazon never properly announced it, it just quietly happened. It's taken a while longer for the Send to Kindle tool to update to add .epub file format support, but people have been sideloading EPUBs over USB for some time now.