The issue isn't really with the routine. That is a whole other subject that isn't really worth getting into. The routine was brought up in defense of why its okay to use those words in standard conversation. My point is that as a straight white male, Lewis C.K. only knows those slurs in the context which he uses them within his routine. He hasn't had someone call him the f-word within the context of hate.
So from viewed purely as a comedy routine, like whatever. I mean, I'm personally not really a Lewis C.K. fan but I don't really have a huge problem with it. There is a place for offensive humor in comedy routines. But comedy routines are still just that - comedy routines. They aren't supposed to be life lessons, and if you try and use it as a life lesson then I think its perfectly reasonable to point out that Lewis isn't really the proper person to comment. Like, if I'm wondering "Is the NFL harmful to its players" and on the one hand I have these actual NFL players saying "Look, I have these brain injuries" and on the other hand I have these people saying "no violence has always been part of the game. plus the players get paid so much and they know the risks", I'm going to side with the players because they have the better perspective. Its the same way here, a straight white man saying how they doing see the issue with the slurs matters a lot less to me than the women, black people, or gay people who *do* take offense with the slurs.