We know that for all intents the headphones are a capacitive load. At 10kHz they might have 150kOhm impedance. That simply means that at 1kHz we expect 1.5MOhm impedance. At 100Hz 15MOhm.You on the other hand, didn't bother to do a 1 minute search on the impedance of the load and ran off with super faulty assumption. Learn what is important, and what is not.
you measured the amps at 1kHz with an impedance of 100kOhm. That’s is 15 times out of spec by your own metric of using a known headphone. You cannot extrapolate the impedance at 10kHz to the rest of the audio band. It is just plain wrong. It isn’t resistive.
The designer assumed a capacitive load. You have measured into a resistive load delivering a wildly different impedance to the designer’s intent across the majority of the audio band. I don’t think this is the designer’s fault.