[jrmg]

But the recipient already has access to the shared document?

Is the concern that the recipient might share the link to the image? Again, they already have access to the shared document if they want to leak it.

I don’t think accidental discovery is possible - there’s a long shard of random data in there. It’s no more discoverable than the share link.

[p337]

If you use the drive integration, you share it with Slack. Slack then creates a thumbnail that is visible in that channel. Imagine pasting a sensitive HR document in the big company chat with everyone in it. No one in the group may have permission via Google, but they can see the thumbnail (and search its contents!) if they have access to the slack room.

Edit: I should note, this is my fuzzy recollection of how it worked 4 years ago when I reported it to Slack. YMMV