Standalone simply means that you don't also need to own the parent game that the mod was originally built off of, which in this case would be Quake. Many authors of such things do include a sourceport along with the release, but not always. For instance there are a ton of standalone Duke Nukem total conversions, but in that community it is very common to NOT include the sourceport as it is assumed that most people playing the mods will have their own copy of eduke set up already. Similarly it is also a common occurance in the DOOM modding community and often you will find "standalone" .ipk3 files which don't require the user to own a copy of DOOM or even FreeDOOM, but you will still need to acquire a sourceport of your choosing first to actually play it.