It is reasonable in this analysis, which is trying to compare normal people to billionaires. As explained in the article, wealthy people can get loans collateralized by their financial assets. Then they proceed to spend and pay back the loan (which is a deductible expense). The result is that they pay minuscule taxes compared to their worth (read more here: https://news.ycombinator.com/item?id=27447959 and here: https://news.ycombinator.com/item?id=27438941). The argument that their worth is not spendable until they sell does not stand up to scrutiny, as the example above shows.

So you end up with these wealthy people spending a ton and increasing their net worth by huge amounts, and paying small (or zero!) taxes.

I agree with you that a different tax law that calculated owed tax like this would not be completely reasonable, but it certainly shows the inequality and I believe it's a reasonable alternative. How would you measure the tax impact on the mega-rich otherwise?