I wouldn't read it that way. The subsequent paragraph is very explicit in saying others can "use, display, and perform Your Content through the GitHub Service and to reproduce Your Content solely on GitHub as permitted through GitHub's functionality (for example, through forking)" (emphasis mine).

Instead, I'd read "copies of Content" as referring to copies produced and consumed entirely through GitHub's services, which can exist independently from the original. E.g., if you have a public repo, someone else forks it, and you delete the original, the other person can still view their fork on GitHub. They can also modify that fork through GitHub's services, e.g., by editing files via the website.