Only because of how we've chosen to define income.
Under such a system — unless I've badly misunderstood something — if a billionaire's assets decreased in value over the course of a year, they would still pay 2% on their assets. I can't think of any sense in which a decrease in the value of one's assets would be defined as income.
I have an opinion of the wisdom of a wealth tax, and I could be wrong. Regardless of my opinion, I think it's indisputable that a wealth tax and an income tax are different and that conflating the two makes a debate on the merits much more difficult.
The "billionaire's tax" Biden proposed earlier this year is closer an income tax that also includes unrealized gains and only if there are tens of millions in unrealized gains in that year.
That said: you pay property tax even if the value of your home declines. It's not that crazy.
I find the article pretty clear, though brief and not very well written. The actual report it's discussing is crystal clear.
The situation is that the wealthy keep getting wealthier without generating any taxable income. Obviously a minimum income tax rate could not possibly solve this problem unless we also redefine income.
Therefore, my conclusion is that a 2% wealth tax on the ultra-wealthy on average wouldn't even keep them from gaining wealth; they would just increase their wealth more slowly.
Even without a wealth taxes there are situations where someone may need to borrow or sell assets to cover a tax bill. The billionaires will be ok.
It is already! It's called the Fed's target inflation rate. Currently, everyone's net wealth is getting reduced at over twice that rate.