It’s not any battery. They just didn’t do the original manufacturing, so you can find compatible replacements elsewhere.
I bought the System76 Kudu laptop back in 2016, but it is actually a W670RZ model laptop manufactured by Clevo Co. in China (unlike my previous laptop which was a MacBook Pro manufactured by Apple in China). System76 wasn’t the only company selling the W670RZ, so they’re not the only ones you can go to for replacement parts.