Because when you buy or sell crypto on a centralized exchange, no actual blockchain transactions take place, it's just shuffling bits in the exchange's local database.
Bitcoin on a centralized exchange cannot be traded for goods and services until explicitly withdrawn, unlike most brokerage accounts where cash is cash and many will even give you a debit card so you can spend directly from the account. This strongly decouples bitcoin and most other cryptos from the total money supply.
When you're on Binance and you send them one BTC as a block chain transaction, you are receiving one Binance!BTC that does not exist outside of that walled garden.
Just look at FTX - some 1.8B in BTC liabilities but zero BTC assets
This splitting hairs over "true" ownership ignores the fact that the original investor analysis done by GP here premised on chain transactions is still semantically wrong.
that still leaves us with "most".
everyone knows the theoretical issues and incompleteness of this statistics collection - since there is no single public or verifiable stat to use - its pretty new to be a person that chooses assume its more likely that people are having distinct addresses on chain, than assume its more likely they're on an exchange
kind of funny how we go full circle here, but okay. hope that helps your reasoning. this article has no way of making its claims either, so there's really no point of appealing to either authority or bolster any opinion.