This rebate is designed to apply to purpose-built rentals at all values. One of the advantages of this is that it creates units across the full spectrum. There are a lot of people who don't move out of their houses, for example, and into purpose-built rentals because there just isn't the type of unit they might need—for example, families needing larger units, or people who are currently overhoused but have no apartments to move into. The idea behind the rebate is to create a wide spectrum of purpose-built rentals that will help meet the needs of a wide variety of people looking for housing.
As you were saying, this opens it up. If people move out of their houses and into apartments, the houses become free. Therefore, the overall supply should loosen and become more plentiful.