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.

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.

permissions might have changed after link preview was generated

(Not the same guy but) I've definitely heard about this bug in the past, but I assume it is fixed now. I can't actually find a reference for it. If I find one within the hackernews comment edit window I'll add it here.

I can't