He starts by explaining why President Biden's desired policy isn't even an income tax:

"The worst of the capital gains tax is that Biden wants to impose a 25% tax on unrealized capital gains, which is definitely unconstitutional because it's not an income tax. … If you haven't realized the gain, you have no income subject to tax under the 16th amendment. It's a property tax. You're taxing property, and property taxes are direct taxes. They're still bound by the rule of apportionment. They're not exempted through the 16th amendment, so it would be unconstitutional."

If such a tax was legal, it would still be a terrible policy, and it'd complicate the already burdensome tax code:

"It would also be economically destructive to try to force people to pay taxes on income they haven't even received. They'd have to sell their assets. Imagine having to appraise your illiquid assets every year and then have to pay some kind of tax without having the liquidity to actually pay it. It would be an economic disaster if that were to be passed!"