A campaign, no matter what you do to it, is just a collection of maps strung together.
Open up hammer, look at Valve's campaign maps, they're the same as any other Source map. There's no advanced scripting or magical coding involved.
A campaign in no way is "A modification of the game itself." By your justification they should start uploading CSS surf maps and calling them mods.
SwissArmyKnife
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