Your parent poster was well aware of that, and also assumed his parent was aware of that as well. His post was a subtle version of "arsenic can't be 'organic' because cannot be composed of carbon chains because it is an element which is not carbon."

The advantage of his version (especially in spoken conversation) is that you can tell someone they are wrong, and typically if they have such a gross misunderstanding, it will go completely over their heads, and they won't realize that you are telling them they are wrong.