It is actually pretty accurate.
Disney sells enough points to have the resort something like 95% occupied. The remaining 5% is not owned by members and is held for maintenance.
Points that aren't sold aren't owned by members and aren't bookable to members on points.
Points used for trading options are sold in order to pay for those options, and not available to members.
So on any given night, the rooms available to members are only the rooms that have been sold to members and not used to trade out. There might be a room that has been held for maintenance that they can pull out in an emergency, but sometimes they've already used that room. In a resort with a few hundred rooms, its pretty much a given that one of them is currently in a condition of limited liveability. (Toilet broke, air conditioning is out, carpet in BAD need of cleaning) - so they don't book to the gills (usually), but darn close, and they aren't going to give you that room because, well, what if someone's toilet breaks.
That means if there is a room available on points the day of, someone isn't using their points.
Now, there are a lot of reasons that happens, everything from "we got suddenly ill" to "we had twenty points left over and it wasn't worth doing anything with them. Timeshares often go unused.