With this question I want to exclude cases where someone is fooled to send BTC to a person (e.g. because social engineering or malware on their PC).

I have seen some arguments in comments to this Seeking Alpha article that claim that bitcoins cannot be stolen if their rightful owner is the only who has access to the private keys.

Has there been any cases of bitcoin being stolen without getting access to the private key?

For example: assuming that there is some exchange somewhere that allows a BTC buyer to manage their private keys outside of the exchange. Could it still be possible for e.g. a hacker to steal the BTC by manipulating the data at the exchange (outside of the blockchain).