Their estimate is based on scaling the estate tax into a wealth tax. The existing estate tax is a wealth tax levied at the time of death. If 2 percent of wealthy families experience a death...each year, then the current 40 percent estate tax should roughly be the equivalent of a wealth tax of 40 percent multiplied by 2 percent — or a 0.8 percent wealth tax.
The IRS reports that for 2017...the estate tax raised around $10 billion from estates over $50 million...Therefore, if this is what the revenue yield would be from a 0.8 percent wealth tax, the implication is that a 2 percent wealth tax would raise a total of $25 billion.
Their higher estimate is based on assuming there'd be less avoidance on a wealth tax than an estate tax.