Are you maybe confusing “downstream” with “mainstream”? Being “downstream” of a product means that you are a derivative of that product (your upstream), taking in basically all of their code and adding your own on top.
Imho, downstream are all consumers/users of that product. I guess it's arguable how a package manager positions in that terminology.
Edit: Also I originally said, "it was the thing used downstream", which just is to describe the consumers. Also not sure what I got into, because the original question is wildly confusing!