Why would she mention "ball game" in this context? Is it a metaphor? If so, is it a common expression?

Native speaker’s answer
Rebecca
Here, "ball game" is metaphorical. It's a very common expression! It means a situation, or activity, usually a very different one from a previous time or an unexpected situation. She's saying that what they do with apparel in their business is very different to what Amazon does. Ex: Night shifts are a whole new ball game to day shifts. Ex: Competing professionally is a different ball game to competing for fun.