We can only identify someone by hardware hash since we already know the (person, hardware_hash) connection. If you already know the person, you do not need the hardware_hash as well to build a profile, nor does the hardware_hash provide additional information.
Orion gave quite a clear explanation on why an IP address is personal data, yet a hardware ID isn't. Knowing someone's IP address gives you a relatively easy way to find someone's location, while knowledge of the hardware id does not point to the physical pc. The only way you can find out which pc a hardware id belongs to is by hacking as many computers as possible and hoping you get lucky.