His theory (which sounds plausible to me) is that a lot of the wealth of the ultra rich is actually bound up or linked to immovable assets. At the same time, investments in these assets is what makes them unaffordable for everyone else (i.e. your kids won't be able to buy a house anywhere in the country, despite them earning more after adjusting for consumer goods inflation).
By taxing large real estate wealth accumulation (think personal wealth in the tens or hundreds of millions and up), you get the best of both worlds: The (ultra) rich who want to stay invested have to pay your taxes (they can't move real estate to a tax haven) and thus transfer part of their wealth back to society. And if they divest from real estate, it puts downward pressure on real estate prices, thus making it more affordable for the rest of us. Win-win, from a society's perspective.
