Earlier quoted context omitted.
Normally most outgoing links that are meant to be private are bounced through a redirection that results in the referrer being a generic Google redirection page. That's probably the solution that was applied here.
But it says this was only a problem for documents that were not converted to Google Sheets etc. I don't think editing everyone's PDFs and every other kind of document that has clickable URLs is the easiest way to solve this.
Native-native docs wouldn't have a referrer because they wouldn't be rendered by the browser, so links would be a direct hop.