Since we got this forum now and this problem is still open, I'm posting the details here. First of all here's the link to the current documentation: http://05command.wikidot.com/licensing:daughter-of-shadows:an-scp-breach-event-game-on-st
Developer was contacted as outlined there and documented here: http://05command.wdfiles.com/local--files/licensing%3Adaughter-of-shadows%3Aan-scp-breach-event-game-on-st/thatonething.png
As you can see at the bottom of that exchange, he's asked a question, and it's one I'm not comfortable answering (though I'll let him know we're working on an answer) without a bit of discussion.
Specifically if a base game contains 173 (and is free), can a developer charge for DLC that does not contain 173? My best guess on this would be no, but I'm no expert.
Also, regardless of the answer on this, it won't be a closed issue until steam updates the page and switches the game to free.