It's not true, even Seedorf said he didn't hear anything, or implied he didn't by his comments anyway. Also, despite Napoli fans being notorious for other things (violence, for example), Neapolitans don't really tend to be racist, that tends to occur more in northern/central Italy.
I'm inclined to believe it has to do with his daughter - he just took a paternity test earlier in the week that proved he was the father of Fico's child. Fico lives in Napoli, so there's a good chance they met. Apparently he wanted to score to dedicate it to his daughter, so was upset when he got subbed.