"Windowing" the impulse response to look at only the initial part shows us the frequency response of the direct sound with little or no contribution from the room.
Yes, but this windowing feature also limits the lower boundary of the FFT. The smaller the time window, the higher the minimum frequency of the FFT, and as pointed out, get low enough and the room is unavoidable.
But the entire premise of ARC is to actually correct for room behavior, so not sure how we got here. We want / need the room in measurements for ARC to work.
Quasi-anechoic measurements ( i.e. simulating a speaker without a room ) is the opposite of what we are looking for here.
Best,
E