You are totally right BlackHatCoiner but those are 2 different things, one thing is the 51% attack, where the miners can modify confirmed blocks, and the second thing is the double spend where the user modifies the unconfirmed transaction.
I think you didn't understand BlackHatCoiner well.
Any mining pool can try to perform a 51% attack and you don't have to own more than 51% of the total hash power to do that. That's what BlackHatCoiner says. He didn't say anything about double-spending.
Question: The dominant mining pool, at the time of writing this, turns evil and uses its hash-power to reorg the chain. A customer of yours sends you a transaction. If you wait for 10 confirmations, what is the chance that the dominant mining pool successfully double-spends / reverses your transaction?