Read the sentence again: the audio is still analog, but there is a digital data stream along with it that is used for station ID/playlist/etc. information. This is different from digital TV, where the entire signal is digital.
That's why it's an in band system, the portion of the signal that an HD receiver responds to is digital. There is a digital signal, it's not an analog emulation. That there is also an analog signal is irrelevant. If the digital data stream is detected, it's decoded, what more do you want to call something digital?
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