Aaron's not wrong to call himself one of the founders. The company behind Reddit was a merger of two startups, one that made Reddit and one that made Infogami, and in that situation the founders of both startups are considered founders of the combined company.
Without knowing the details about this particular case that sounds wrong.
Note how PG subtly shifted from „Aaron co-founded Reddit the product“ (what everyone meant) to „Aaron co-founded the company that today owns Reddit“, which is defensible, but mostly uninteresting.
Aaron's not wrong to call himself one of the founders. The company behind Reddit was a merger of two startups, one that made Reddit and one that made Infogami, and in that situation the founders of both startups are considered founders of the combined company.