You're proving the parent commenter's point that "this is about availability of special features which require a dedicated driver non-Apple devices are not expected to have", because there is no standard way in BLE to report more than one battery value. Wireless earbuds are a device pair, each with its own battery.

Apple, like every other vendor, does not have a choice but to implement this as a proprietary characteristic. Pre-BLE, other vendors copied Apple's de-facto `HFP AT+IPHONEACCEV` standard for reporting battery levels to the OS.