Apple could have employed some of their fancy new MacBook terraced battery technology and built in an antenna extender to prevent signal loss. I would have loved to have seen a case as large as a Mophie that blew it away in capacity. Can you imagine the buzz that would have been generated if it had a week of battery life? But that may have doubled or tripled their cost considering the ten month window Apple is stuck with.
Looking at the battery picture in the iFixit tear down photo, it looks to me like they may be using a “parts bin” battery. The industry is probably awash with vendors who make a battery in this size and they’d all have already paid for development costs on other projects. Apple could send their specs out for bid and let them undercut each other to win the business. The material cost of the battery would be much less than one that they developed on their own and the size of the battery would prevent any signal loss. From a practical point of view, it makes perfect sense.