I'm not sure what point you're trying to make here. If more people sell fiat and buy crypto, money is going "into" crypto. If more people sell crypto to buy fiat, money is coming "out" of crypto.
Their point is that thinking of money going "into" crypto means that the money is there to come out of it. And that's not generally true.
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)
The money going in is the difference in price between buy and sell. If you buy for $5 and sell for $100 that is $95 of inflow. If you take your $100 and the buy another $100 worth, that was previously only worth $5. That's another $95 of inflow, but if you bought your original or it's value equivalent back for $100 it is not a new inflow. With traditional assets this is easy to determine, with crypto it's nearly impossible.
There are X bitcoin and Y ether out there in circulation at the moment. All are currently owned in a definitive sense because blockchain ledger.
When someone sells fiat and buys crypto, that implies that at the same time there is someone on the other side of the trade buying fiat and selling crypto; right?
In reality this is devaluing existing crypto relative to other assets. The OP still stands in that money does not flow into crypto. It's not a black hole. Fiat money is being traded for an asset between two parties. The other party gets the fiat.
In other words, you can't create or destroy USD (for example) through a cryptocurrency. You can only affect the cryptocurrency itself. Otherwise we'd have a large problem on our hands...
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)