Yeah agree with you that its quite clear whats happened here, even without actually having Firmino on camera mouthing the N word.
You can see that Holgate doesn't react to what's being said to him initially as its in Portuguese. But then something is said that has caused Holgate to react - if you look carefully at this exact moment the ref has turned away from Firmino to look at Llana so hasn't seen it and it he won't necessarily have heard it either given the noise levels from the crowd and other players around him.
Holgate immediately turns to the ref and tells him he's been called the N word. I doubt very much that anyone would think to make this up on the spot and for one, Holgate wouldn't make it up when he would expect a camera to pick it up, no way does he know that his team mate hasn't blocked the only camera angle that would prove his accusation to be true.
All the evidence points to him using that word - absolutely disgusting.