Nobody knows, but my guess: Apple is hiding a test of advanced battery fault detection and monitoring technologies that will eventually show up throughout their battery-powered products. Final Boss Lightning was the simplest, least risky way to support the power delivery and communication needs in the AVP Mark I timeframe.

https://patents.google.com/patent/US8986866B2/en?oq=8986866

They don't want you to use a replacement battery they didn't sell you

If I had to guess, one group came up with (reasonable) requirements for this connector to handle certain currents and voltages, be reliable, be nice-looking, and be proprietary for the purpose of not dealing with RMAs related to people connecting shitty usb-c batteries directly to the headset. And another group realized "hey, we have all kinds of documentation, test results and tooling, we can make a wider lightning cable with low risk and low cost. Sure, we are moving away from lightning but since this is internal to 2 apple products it doesn't matter."