The product designer must be good at creating usable prototypes
A good product designer must be good at creating usable prototypes that can be put in front of potential customers. It is not just "design" but interface, interaction, usability, and understandability -- all with the necessary speed.
Product designers must be proficient with building functional prototypes of potential product features. They are not "graphic designers" because their design is meant to be used as a simulation of an actual product, without any actual coding.