To be exact it doesn't need the sources, it needs the function signatures and type definitions so the equivalent of C header files. If you don't want to share the full sources with your library you can generate those header files automatically using the -H flag in dmd. It will produce a "D interface" file with a "di" extension.
But - if the library was open source, it would be better to just share the sources than a compiled file? (In Java we share/use the zipped JAR file with compiled classes).
