It provides as much security as you have trust in the people you share it with. If I set one of my documents to "anyone with the link" just so that I can access it when I'm not logged in, and I'm careful to not share the link with anyone else, then I'd argue that it's still provides some security because I trust myself.
If I shared a document with only you (directly, not using the "anyone with the link" permission), I couldn't stop you from taking a screenshot and sending that to whomever you pleased (or printing them out and leaving them in a conference room, like you suggested). Sharing documents online in general can be troubling if you don't trust the people you send them to. It's easier to share a link than share a screenshot, though, and a link would provide continued access going forward, and could remove deniability that the screenshot was faked.
But without authentication, there's no way to know if the people who claim to be the people one shared the link with to be the people one actually shared the link with.
Whole-document transformation and copying are a concern, but I think that's usually treated as a different category of issue from "The server I trust to store the data securely gave it up to some anonymous person who passed a correctly-formatted request to it because the server can't know any better."
Yes, certainly this particular bug was serious and a real problem that needed to be fixed. The link should not be shared accidentally like that.
I was just making the case that "Anyone with the link" still provides some security. Just like you don't know if the people accessing the document are the ones that you shared the link with, you don't know that the people you shared the document contents themselves with aren't showing others without your knowledge.
If I shared a document with only you (directly, not using the "anyone with the link" permission), I couldn't stop you from taking a screenshot and sending that to whomever you pleased (or printing them out and leaving them in a conference room, like you suggested). Sharing documents online in general can be troubling if you don't trust the people you send them to. It's easier to share a link than share a screenshot, though, and a link would provide continued access going forward, and could remove deniability that the screenshot was faked.