Perhaps I'm explaining it poorly, that's entirely possible. The frontier is where land value is zero, at the margin of production. It is possible to get value out of the land, but only with hard work. Currently this land may be held without using it for production, but it largely doesn't exist. This is because the lack of a land value tax incentivizes landholders to own all land for the purpose of speculation rather than production, even if it's economic rent value is low. With a land value tax, people who hold land purely for speculative sales prices would sell it because the sales price would drop to zero, therefore freeing up those pieces of land for families to move to and work upon. The frontier, the margin of production, where only hard work would make the land valuable, reopens and other land also becomes more mobile because there is a disincentive to holding land for speculative purposes, reducing the price of housing in general and contributing to housing liquidity.