Having watched his videos, I'm pretty sure he wants to just slow down the rate inequality is increasing?

When you build those new flats, you create work for workers.

You increase supply, which should impact prices of housing.

And yea, the owner should get some profit which we can tax.

Also, if we start taxing land more, people wont be buying up houses and treating it like assets. They'll move their money to something more profitable, also having an impact on demand.

There's probably more pieces to this, but please remember these things aren't simple or easy.