It uses USB-C for the charging cable. The cable between the headset and power supply is a locking proprietary connector, discussion on hackers news indicates it’s because USB-C locking connectors are not elegant and it has specific power requirements so using it with a random cable and random battery pack is not a good idea. The “battery pack” is also the power supply for the processor, it’s not really locking into a proprietary cable any more than it’s locking you into a proprietary operating system. Saying either of those is essentially meaningless to the discussion of the device.

They didn’t say how thick the batteries are. So saying something like “the size of an iPhone” is really not a good way to measure things. The important information here is that it is a 3-cell pack with ~35Wh.