My understanding of the article is that the Moores are suing probably based on the difference between wealth and income.
The Trump policy stipulates that if you own 10+% of a foreign business, you are taxed on the value of that ownership. Valuation is a function of wealth as I understand it because it's folded into how much a person is deemed "worth." Something similar here is property or stocks. If you own so many shares of a company, you are "worth the value" of those stocks even though that isn't currency at your disposal. That's why people are taxed on the value of stocks they sell because now that's moved from an imaginary realm of value into income into the bank account.
The Moores are saying: but valuation isn't income, so you can't tax us.
The lower courts have said: valuation is deferred income, so yes the federal tax on you was legal.
I don't think an argument about discrimination is at all on the table. I think it'll be a more pedantic argument of value, wealth, and income - which of these is taxable and which of these are imaginary (with real outcomes).
This is a complex notion for a non-legal person such as myself. The Trump tax package was absolute bullshit, but even I am surprised this one provision was in it. Surely this was created to hurt people Trump deemed as enemies, such as Mitch McConnell's wife Elaine Chao, but then "ordinary" rich people like the Moores got swept up into it. I am very much in favor of more taxes on the rich, especially for overseas finances, which often is sheltered from federal taxation and transparency. But even I am torn about trying to make the distinction between valuation and income, and which should be taxable.