Actually, your cousin might be correct. If he just pre-ordered, then it means that he pre-ordered when the pre-order allotment has been exhausted. After the pre-order allotment has been used, those who pre-order will get their phones two weeks after they pre-ordered it.
I think it's because Apple is expecting a shortage. Even if they say it would be available on the 19th, there is no guarantee that there will be phones available on that date or any day after. So if you really want it, then it's best to preorder!