Yes, the battery is replaceable by user and is not soldered in. I had emailed Fiio Service and they sent me a video on how to open up the Fiio X5. However, if you open up the Fiio X5, warranty is voided. If you ask Fiio, they will mail you a battery for a fee. I had the same concerns as you.
This is great to hear. Could you DM me a link to the vid?