It's fairly easy to agree that this action is not enough to get Uber off the hook and can indeed be thought as a PR stunt. But why do you think Amit Singhal's race is a factor here?
https://medium.com/@amyvertino/my-name-is-not-amy-i-am-an-ub...
It seems racism is another aspect of Uber's culture. Since the narrative is Kalanick sets the culture, and he chose to fire the brown guy who has no apparent complaints from Uber co-workers instead of firing the white guy mentioned in Amy's post, who has actual complaints against him, it would seem firing Amit was a racially-motivated choice of scapegoat.
He's obviously hoping to feed this into the news cycle to make the negative attention go away so he can resume business as usual with no real change. My point is if he had fired Amy's perpetrator it might have worked. But with her adding allegations of racism to the mix, this stunt fails.