HD always has been more of a buzz word than a hard technical term. HD in apps generally mean it up scales better than some made for phone apps and supports less blur when you zoom in on certain sections (this is more of a software fix obviously). The kindle fire does have a Pixel per inch of 169 I believe...its not astonishing but its not horrible...
HD is used all over the place for a variety of different actual viewing experiences....notice on many download sites the "HD" video varies a lot
http://att.macrumors.com/attachment....7&d=1201708357
1024x600 on the fire.
I think HD is just an app buzz word for "made for a screen larger than your average phone". The Apple app store does the same thing even though the Ipad isn't true HD as we think of it in T.V. terms...it means the same thing there.