I might be missing something, but that link really only talks about the potential benefit of a file already being cached locally on the end-user's device.

There's still a significant benefit in just storing the file closer to the end-user, rather than in a single central location, as the CDN is likely to have both lower latency and have higher bandwidth than the source website.

Now, if your file was so unique and rarely accessed that it wouldn't even be cached on the CDN, then I'd agree with those findings.