In GitHub, fork networks are represented as one repo on disk (this is the main optimization that makes community pull requests possible at all). So you could fork a repo and then push an object with a hash collision to it to change a file in the original, in theory.

(In practice this is harder, as the article mentions, because the new forged object would have to be a valid gzipped git object of the same length. And GitHub probably knows about this type of attack and might just, for example, prevent existing objects from being overwritten)