This means there will be no punishment towards Cala as there is no evidence of racism and ultimately the case is now closed. Do I believe Diakhaby, yes, there is no reason why he would lie and his reaction in the match seemed genuine, but again without audio of the incident we can not know for sure what exactly was said and if Cala is a racist or not. Plus there is the issue of being an asshole, a jerk who wanted to trigger Diakhaby without him actually being a racist. These are complicated cases and again without audio evidence its hard to say what the truth is.
Ultimately I think this is the right decision, innocent until proven guilty and since there is no evidence of the incident, its claim against claim and thus you can not make any sort of judgement based solely on that.