I think you are under a misunderstanding. HD radio is NOT "digital". It is a proprietary format analog signal with a digital adjunct. See: <http://en.wikipedia.org/wiki/HD_Radio>
Both IBOC signals are truly digital although on AM, it's a Hybrid Digital system. Please note that your quoted source to rebut the claim that HD is digital states:
"HD Radio is the trademark for iBiquity's in-band on-channel (IBOC) digital radio technology used by AM and FM radio stations to transmit audio and data via a digital signal in conjunction with their analog signals.'
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