Is restoring iPhone as new from iTunes equal to erase contents and settings on the iPhone

I found different answers to this question. Some says the former creates a new key thus making all previous data inaccessible. Someone else says only the latter erase the key as in this discussion:

Does erasing all content and settings prevent people from being able to still access your private information if you sell your phone?

I sold an iPhone to a stranger and I can't understand if I did everything correctly by using iTunes.