They're not "BD-As." They're just BD-Vs with limited or no video content. It's not like DVD-Video vs. DVD-Audio, where the latter uses a different folder structure (AUDIO_TS) allowing playback of audio with no video content. All audio Blu-rays require at least blank (black screen) videos to play back audio, which is stored multiplexed with (attached to) said video. The proposed BD-A standard would have followed the same conventions as DVD-A, storing lossless audio in a separate folder from the menus and video content, and allowing for audio-only (or audio-with-static-image) playback. Presumably, it was never implemented because BD-V already supported lossless audio (up to 7.1 and beyond), and the slight "disadvantage" of requiring video was deemed inconsequential.
HFPA (and the similar, still-extant Pure Audio) was a marketing term, nothing more.