Why not? Are you talking about the game executable or the game-logic "scripts"? The former is definitely runnable, the latter is runnable if you compile it as a dll rather than as bytecode (using the special compiler).
I tried to do everything as much as possible like the tutorial stated. So I opened the VC++ project that comes with the "quake source download", and opened it. When I try to run it, it asks for an exectable (which should be quake3.exe -> according to the tutorial), but when I then run it, it wont load since it cannot find the files it needs (files from the map baseQ3). This is not so weird since the source is not in the same map as the executable. When I copy the quake files to my source map, i am able to run the game from VC++, altough it seems like it is just running quake3.exe, and the just compiled source is just ignored, since changes in the code are not visible, and breakpoints that are set, do not get triggered.