This may be unfair as Aaron isn't there to give his side of the story, but the way I understand is that the YC put these 1+2 people (who were all talented) together to help build reddit, but Huffman and Ohanian were far more interested in building reddit as it was originally their project and they had been working on it for 6 months, and it seems like Aaron wasn't as interested in it.

So yes, while Graham did forcingly merge these two together, I don't think it necessarily makes Aaron a co-founder and contributor.

EDIT: Just to add, Aaron's project was "infogami" which seems to be a CMS system, and I'm not sure how much of that actually contributed to reddit. It seems like his project was mostly scratched and the project that Huffman & Ohanian were working on ended up being the real success.