[kevincox]

If I am understanding what is happening is that the sender has access to the document and via the Google Drive plugin for Slack they are effectively attaching a preview image when the share the link.

So the stance that Google is taking is probably taking is that this is equivalent to the user sharing the link also sharing a screenshot as a preview. (Something that they have permission to do)

I do agree that this may be a bit surprising but the sender has "opted-in" to this behaviour when they signed into the Google Drive plugin for Slack. But I don't find Google's perspective completely unreasonable.