Source available doesn't mean that you can't do anything with the code, it usually means that there are limitations (e.g. sharing it) with what you can do with it.

There are a lot of cases where "source available" expects you to modify the code and even share patches with others. A popular example nowadays would be Unreal Engine 4, but it isn't anything new - Borland's OWL was distributed as source code and you need a license to use it (it is part of C++ compilers so any of their compilers would provide a license), but over the decades there have been improvements made by its users in the form of OWLNext that is meant to be installed "on top" of OWL (ie. wont work by itself).