To clarify, I think what they mean is that it's possible that those old links have already been compromised, even though none of the documents (new or old) are vulnerable anymore.

The bug has been patched so that any document with the "anyone with the link" permission will no longer leak its location in the referrer when someone clicks an HTTPS link in it. But it's possible that that happened in the past and the link was already leaked (and unfortunately, they can't exactly fix that). So if you have an old document, it's not vulnerable anymore, but it may at one time have been vulnerable so you might want to update it to have a new link (following the instructions in the Google blog post).