This is not correct. HGV absolutely can, and does, rent inventory controlled by HGV on Hilton.com. You can rent most, if not all, HGV properties on Hilton.com. Here are several categories of units for rent in early January at Lagoon Tower and Kalia, for example:
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These are the actual rates, roughly 30 days out (just like Open Season). As you can see, Open Season is still a better deal.
When someone rents an HGV unit on Hilton.com, they get the same benefits as they would with a hotel, like daily room cleaning, even though it is a timeshare unit. And in most cases, Hilton does not own the hotel rooms they rent out on Hilton.com either. The Hilton hotels themselves are almost always owned by REITs, investment firms, etc. Hilton Hotels Corp just acts as the property manager and/or reservation agent. It's similar for the Hilton Grand Vacations units that are listed on Hilton.com. Those are rented out on Hilton.com by virtue of the marketing agreements that HGV has with Hilton Hotels Corp.
While you are correct that a property like Lagoon Tower is largely sold out, HGV still winds up controlling inventory at virtually all of their resorts - when owners convert their week to Hilton Honors points, when sold units are reacquired through owner upgrades, ROFR, or foreclosure (until resold), etc. etc. That's how they get inventory to rent on Hilton.com.