kobe is on the verge of retiring, with his injury there isn't much testing he could or should be doing if he wants to have a chance of still being relevant in the upcoming season. they should consult with someone else
Kobe is probably going to retire soon but that doesn't make him not relevant in the eyes of the public. He's still known around the world and even if he isn't at all star nba level anymore, hes still a great athlete and is still better then 99% of the world at basketball. Retired athletes, especially recently retired ones are great for product endorsements in the field they were in. Jordan for quite a while after his last retirement still endorsed products that showed him playing basketball. Jordan still has a large presence endorsing products and while Kobe is not as big as Jordan, Kobe can still bring a lot of people in to buy x, y or z.
I'm sure they are consulting with other athletes. Maybe not as big of a celeb as Kobe and apple is gong to test the watch with athletes from different sports as well.