Everybody, please, repeat after me:
"The bitcoins I have on an exchange are not my bitcoins. They are an obligation of the exchange to pay me back said number of bitcoins."
If the exchange gets hacked and loses its bitcoins, it cannot meet that obligation, and you will have no bitcoins to withdraw. Hence, they were not and are not your bitcoins, they belonged to the exchange.
It's like having money in a bank without deposit insurance. It's not actually your money, it's a bank's obligation to pay you back the money on demand. Will they be able to meet that obligation? Who knows.
They are still
your bitcoins, because they can be used as them on demand. That's like saying the money I have at the bank is not my money. Really, all money is just an obligation of someone to pay me back for something.
I don't think you read what he wrote very carefully. In a simple practical sense, a bitcoin is yours if you control its disbursement. If an exchange gets hacked and all its bitcoins stolen, and you had bitcoins on the exchange, you no longer have the ability to disburse those bitcoins. In other words, they're not yours anymore.
And, as runeks pointed out, there's a big difference between money in a BTC exchange and money in a bank. Typically you have some state backed deposit insurance with the later. There is no such infrastructure in place for bitcoin, and there likely will never be.
