The easy-to-use Bluetooth-enabled stereo speaker accessory own 1000 mAh internal battery and kickstand.
http://www.notebookcheck.net/Harman-details-JBL-SoundBoost-MotoMod-for-Moto-Z.167846.0.html
I think your article is wrong. It does not have Bluetooth. JBL/Harman never state that, instead they are touting this as an alternative to a Bluetooth speaker.
The internal battery has nothing to with Bluetooth, the battery makes sure that the phone's battery does not get drained., that's it.
http://news.harman.com/releases/harman-and-motorola-introduce-jbl-soundboost-moto-mod-for-moto-z (http://news.harman.com/releases/harman-and-motorola-introduce-jbl-soundboost-moto-mod-for-moto-z)
https://www.motorola.com/us/products/moto-mods/jbl-soundboost-speaker#moto-mods (https://www.motorola.com/us/products/moto-mods/jbl-soundboost-speaker#moto-mods)