I probably wouldn't have posted this if I had received this explanation from Google. :)
This was their response: "Hi! We've decided that the issue you reported is not severe enough for us to track it as a security bug: when someone with access to a doc sends a link over slack, they express their intent to share this document, hence the preview shared independently from the sharing setup on the doc does not represent a significant risk."
It is not intuitive if you are not used to how security really works, which is that permissions are granted to masks that you wear, not to any real person. When you give slack permissions to use the drive integration, you are giving them a copy of your mask, and they are you, even unexpectedly. You have a (subconscious) reflex to automatically create and share screenshots of documents that you link, that you have trained via slack's hook. Remove that integration, and everything works as expected.
I agree that it's poor behavior and potentially could be part of a critical security compromise. But you'd need to get a hold of the right team. The Google Docs core engineers probably don't care and couldn't fix it anyway.
I don't know for sure (and can't check now since I no longer work there), but my impression at a former job that made heavy use of Slack and Google Docs was that in a corporate setting with Google Enterprise or whatever it's called, the Slack integration was far more cautious about showing previews. It even alerted you if you posted a document that not everyone in the channel could view, and gave you the ability to grant them access right then and there. IIRC, previews were hidden if you didn't have access. I don't know if that is a different plugin or just better behavior in a corporate setting.