Writing music is writing. It is no less and no more than lyrics writing. In fact if you only produce music it is an instrumental, if you only produce lyrics it is lyrics writing. To qualify as songwriting it has to include both processes. If you are a lyricist with no music skills then you need to collaborate with a music writer to produce the finished work: a song.
The reason is that a song is more than adding lyrics to music or vice versa. When you add the two together the two parts work together to become something much, much stronger. Sure in some genres music is less important, or in others the lyrics are less important, but in both cases the combination is still vital for the overall effect, the overall connection between writer, artist and listener.
So in my opinion working on the music and working to combine words and music into a song, on your own or with a collaboration partner, is absolutely vital.