And if cashmere isn't pure, than is it just wool?
No. Cashmere is the undercoat and good cashmere comes from breed/region-specific animals. The longest and finest fibers of the coat in most instances. Many "cashmere" products are spun with top coat, or worse synthetic materials, to imitate the strength or hand of true luxury-caliber cashmere. So to call cashmere "pure" only means that it hasn't been spun with other fibers. In this case 5% of the garment is made of 100% cashmere fibers.
But if pure cashmere is only 5% of the blend, how can it be pure?
As stated above, the "pure" only refers to the cashmere used in the jacket and not the jacket as a whole.