It's literally SDK, if you care to actually read your link. Not the engine itself: the game-specific logic DLLs (client.dll and server.dll in the game's folder) that hook into the engine, and can be modified to make mods. Valve started publishing those with the release of their first game in 1998.
If you are familiar with the latest version of Unreal, then it's like having access to the "Games" project in your solution and only some headers from the "Engine" project.
You can read more about the modding workflow here: https://developer.valvesoftware.com/wiki/My_First_Mod
If you are familiar with the latest version of Unreal, then it's like having access to the "Games" project in your solution and only some headers from the "Engine" project.