Unless something has changed since less than a week ago, if you try and connect to Steam while Steam is down for any reason (say, a DDOS attack, like in this article), you will fail to authenticate and be left in a "logged out" state. At that point there's no way to activate offline mode because you can't connect.
If you were already logged into Steam and attempt to "go offline" it will attempt to authenticate with the Steam servers, and again - if Steam is down, that's the end of that.
This happened less than a week ago. That's not misinformation, that was me trying to open Steam on Saturday to check out the holiday sale.