Apparently, the Clip only recognizes Audible audiobooks under the player’s separate “official” Audiobooks category listing. It would be great if this category could be generalized more in the future in a firmware upgrade, to cover non-Audible audiobooks as well, either those placed in an Audiobooks folder or with the term Audiobooks in the genre tag field.
The workaround for now is to use the term Audiobook in the genre tag field–and this should be singular, not plural, as the player does not seem to recognize the plural form Audiobooks as a genre category and categorizes those as Unknown. The downside of this workaround is that it throws the audiobooks chapters in along with music for playing, such as in shuffle mode or when playing by title.
If there are other suggestions, feel free to throw them in the pile.