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.
Water is considered an important part of organic chemistry by everyone, but pedantic chemists. Original poster may have been thinking of cyanide but As is often found in plants and animals and is not man made. Which fits the casual definition.
Arsenic is an element. Elemental arsenic, as with elemental carbon, or hydrogen, or any other element, is made of protons, neutrons, and electrons.
Its chemical properties are defined largely by its valence electrons, and atomic mass.
There is no more carbon in arsenic than there is sound in light. They are different things.
https://en.m.wikipedia.org/wiki/Arsenic