As I understand it, writing involves the ULA 'switching' its inputs to outputs and driving the same (stereo) pair of coils with the 50/100KHz signals.
Yes, I agree ZX8302 ULA does FM coding / decoding, while MD ULA does FM amplification and TTL hysteresis correction when reading, in my opinion. As I think ZX8302 needs TTL signal levels to safely trigger transition states, you describe during read, while signal transitions should not also be slower.
When looking at `waveforms' in SM, I can see 2V and 1V in samples, which exactly hits TTL forbidden zone. That possibly means I do not understand it.