They are talking total available market share and only for notebooks, not a subset of it...frankly, most pc notebook manufacturers could give a rats ass about musicians and artists. They don't make any money off of them, especially when they make ~$15-25 a notebook, and you have big businesses buying hundreds of thousands of notebooks for their employees. If apple competed in the fortune 500 business area in light of laptops, then the market share would be higher.
It would be interesting to see the market share mac and pc notebooks have just for musicians, however. Any guesses? I'd say 75% mac.