Soft-spun is a marketing phrase for any fabric that is soft to the touch. The author probably isn't a specialist in fabric, but they have heard of a phrase like "soft-spun" or "spun soft" or something, and that is what they have used.

Is soft spun the same as ring-spun?

There probably isn't any difference in the quality. Ring-spun has a slightly higher edge to soft spun. Ring-spun will not pill or get fuzzy.

Does soft spun cotton shrink?

Combed cotton is an extremely soft cotton because the cotton fibers are specially treated before they are spun into yarn. This type of cotton is also more expensive. Combed cotton will shrink if it is exposed to heat. This means it should never be washed in hot water or dried in a hot dryer.