Possibly they refer to this:
https://news.ycombinator.com/item?id=39172527 “I received a link to a Google doc on slack recently, but the owner had forgotten to share permissions with me. Though I couldn't view the doc when I clicked it, I did notice that I could view the first page of the doc in the link preview. It was very high res and I could view the text clearly.”.
If so, pasting that link into Slack may reveal its first page.