More detailed teardowns show that the adapter and the bundled earbuds are using the same DAC/Amp chip (same design and part number), it appears to be an as yet unknown chip with an Apple part number so looks to have been made specifically for these devices.
Perhaps the use of the same chip in the adapter and the headphones explains the EU power limitations being enforced with the adapter despite the fact that it didn't actually need to be enforced.
Not all adapters are created equal, some of the 3rd party non-MFi devices available on eBay and Aliexpress state that they don't work with the Mic function suggesting they missed out the ADC bit; this further suggests that they may be using inferior components.