[Someone]

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.

[necovek]

It could even be a Google docs Slack app that has the bug of generating a preview if the sharer has permissions on the doc (and they usually do) and the preview generation enabled.

[areyourllySorry]

permissions might have changed after link preview was generated