Dark Messiah isnt actually a mod, its a full game
Yes, technically it's not a "mod", but the point was it uses a modified version of the Source Engine rather than its own; the Source Engine would have been purchased under licensing acts, which allows the game to be sold for profit, rather than merely distributed freely (as a "mod"). In the same way Half-Life is a "full game", but using a modified version of the Quake Engine.
"mod" is a lose term in regards to game development (as are many coined phrases in this young industry), but it has an unwritten context that a "mod" is free and under the effect of the basic user software license terms of agreement. When a game engine is licensed, that copy is then private and seperate from the original franchise. Thus it is its "own game", but the engine in question still belongs to the original authors. The game will always be a mod of the Source Engine, even if it's not under the Valve production label.
In the case of CS/CSS, TFC, DMC etc. They were produced by the very same software developers that made the original engine. Thus it is a part of the same franchise, but still a "mod". DoD was originally built as a public "mod", but was later baught out by Valve as an official part of the franchise.
Terr's point was that some might consider the use of a game engine, developed by someone else, as cheating in the development process. But under a technical and legal sense, that material was purchased (or rented if you will), and will never be claimed to be authored by the users (new development team) of it. For example, Dark Messiah brandishes the Source logo trade mark, as part of the licensing agreement.
IE: Why do we consider the licensing of game engines acceptable, but not the custom revised official maps as legitimate creative productions; worth (supposedly) equal respect.
But the two concepts are in 2 different leagues, and 2 totally different legal and social contexts.