ProPublica said that these billionaires were avoiding taxes because their unrealized gains weren't being taxed, and implied that they should be in a direct sense for fairness.

(Note that ProPublica mentions wealth taxes at the end, and then immediately dismisses them. Their object here really was/is gains taxation in a direct sense.)

My point was that this isn't tax avoidance, that there are reasons we don't tax unrealized gains directly, and that virtually no countries do it that way.

It's true that a few (and very much declining) number of countries have some form of wealth tax that includes some amount of unrealized gains. The Netherlands, for example, marks-to-market on Jan 1st of the tax year then doesn't actually track gains/losses over the next 364 days. So that obviously isn't a direct tax on gains. But it's certainly adjacent, and I said in the parent comment here.