It comes with a built-in screen protector. You don't need to add one since it already comes with one. If it didn't come with one, you would add one. So, either it comes with one built-in or you add one. What's the difference?
I'm not sure what their specific claims are or what their guarantee is, but it does have a built-in screen protector. I'm not sure how resistant to scratches the rest of the case is. I definitely would not put it lose in your purse with other sharp things like keys. Maybe you have a small pocket inside your purse it would fit snugly in?