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Not sure what your link proves. In reality, no one knows if this statement to be true because addresses and wallets can be shared. There was a Bloomberg article with lots of interviews on this topic with people in crypto business and no one seem to deny this: https://news.ycombinator.com/item?id=15877838

Many of these wallets are actually exchange cold storage wallets, so they hold the BTC but not per se "own" it

In practical terms exchanges do own these bitcoins in that they a physically capable of trading them.

You'd probably find that 40% of US equities are owned by fewer than 1000 asset managers as well, so I'm not sure that this is all that relevant. If Vanguard decided to liquidate their holdings overnight there would be pandemonium just as if these people decided to liquidate their BTC, but it's not going to happen in either case.