It's a tax on wealth, but as the Krugman article points out it also allows real wage cuts without the psychological barrier facing nominal wage cuts, which facilitates corrections when wages have overshot their sustainable levels, assisting in reaching full employment (unsustainably high wages necessarily reduce employment levels.)
This is related to why minimum wage is inferior to UBI as a basic support: minimum wage is in tension with employment, especially at the low end, in a fairly direct way. UBI, while it had to be supported somehow, isn't directly in tension with employment specifically at the bottom.
I'd like to see minimum wage tied to inflation but UBI tied to a funding stream that grows with output, so that per capita benefits grow (over the long term) with per capita output. And I'd like to see minimum wage decreased from the level it is calculated at with inflation adjustments by the hourly wage equivalent of UBI (for simplicity, annual UBI/2000, so that a $12,000 annual UBI would reduce minimum wage by $6/hr from it's pre-UBI level.)