While I can see how the delta in this metric is suggestive, it is not clear how it affects the available housing stock (rent+buy).
What I think people actually care about is unoccupied investment properties, where someone buys and leaves it vacant. Apparently a lot of Chinese capital is parked in this way due to their capital controls preventing cash from being freely moved out of the country, though to be clear it’s not unique to China.
Or if these properties are being flipped, perhaps that has a short-term negative impact on available housing stock as properties are taken off the market while being renovated. (If the marginal second bidder would have moved in to the house as-is, then the winning bid renovating and selling is reducing supply / increasing demand for housing.)
As you note, a whole other kettle of fish around REITs that won’t use mortgages. In the US, REITs are a major contributor to new construction for example.