I'm not sure how long ago that part of the article was written down, but the Master Levels for Doom II is available as a standalone download on Steam for some time now (I have no idea when it was put out on Steam outside of bundles) for $14.99 USD, and when I purchased Doom II during the Steam Summer Sale of 2015, the game came with the Master Levels with it. I don't know if there are any other sources of customers also getting the Master Levels with Doom II included with the purchase on Steam.
Topic on Talk:Doom II: Hell on Earth
I checked the Steam page for the Master Levels and I found no option to buy it standalone. Can you give a link to where this standalone option is available?
If you mean as a separate downloadable entry in your Steam library once purchased, then that has always been there.
I think this should be the standalone release of the Master Levels on Steam: http://store.steampowered.com/app/9160/
-EDIT- Wait, I just noticed the Add To Cart option is only in bundle form. Whoops. I should have looked at the page clearly before asking. Should we mention that the Master Levels comes with Doom II since that's what happened with me when I bought Doom II by itself?