I don't think the spot is as black-and-white as most fans think as far as winning that challenge. It's hard to tell when the foot might have touched out of bounds or the knee hit the ground. I think they got a crappy spot, but I'm biased. I wouldn't call it indisputable.
Also, why bother using a challenge in preseason? I know they keep score but as a coach you're not really trying to win in preseason, it's primarily about evaluation for the coaches (and avoiding injuries).