This is correct. The unit doesn't enter into HGVC club inventory until someone purchases it.
There were a lot of people complaining this summer as they could not trade into HHI via HGVC but could pay and get it via Hilton.com. People complain about the same thing in the Lagoon Tower too from time to time. Mostly newer HGVC owners.
It is either not HGVC inventory to begin with, or owners have opted to convert their week at HHonors points or something else, and to make money on it, HGVC rents it via Hilton.com. In either case the unit is NO Longer available as a HGVC booking, so complaining about it is not going to help you get into it via HGVC points.