When I buy a BTC, my dollar goes to someone else bank account, who then does whatever with that dollar (spends it on electricity, perhaps). If I later get out of BTC, I don't necessarily get my dollar back. So if $1B has gone "into" BTC, perhaps only $1M comes out of it later. So seeing there as a stock is misleading.

That said, if you trust the maintainers of stablecoins, then the article's assumptions do hold. $1B spent on USDT and then later reversed should result in $1B coming back. So you can think of stablecoins as a stock (relative to the tethered currency at least)