No porridge is not a soup.

You cook porridge by boiling off water until you hit the right amount of water absorbed into the cereal, and it not being soup or gruel like. The starchiness also plays a role in consistency.

Porridge does not use or create a broth, soup does.

Porridge is always cooked until there is no or little liquid left, you can’t have soup without liquid.

I am willing to entertain whether nothing that porridge can be a stew, but it is not a soup.