On the home page there is a question that reads, “What will I need in order to play?” Followed by the explanation that one must have a game featuring the source engine. I recently bought Half-Life: Source, and I was wondering if that would apply as one of those games, even though the “detailed list” did not list it?
If you can install Source SDK (found from Tools in Steam), then yes, you can play it.
Half-Life: Source is not one of the games that has the Source SDK, therefore, no.
Go by the list.