In Philosophical Investigations W. says:
"But in a fairy tale the pot too can see and hear" (Certainly; but it can also talk.)
But the fairy tale only invents what is not the case: it does not talk nonsense. - It is not as simple as that. Is it false or nonsensical to say that a pot talks?"
"What gives us so much as the idea that living beings, things, can feel? Is it that my education has led me to it by drawing my attention to feelings in myself? That I recognise that there is something there (in me) which I can call "pain" without getting into conflict with the way other people use this word? -I do not transfer my idea to stones, plants etc.....
...Only of what behaves like a human being can one say that it has pains."
So W. might be saying that we cannot say that a zombie experiences pain, it is either false or meaningless. But I think that depends on whether we judge a zombie to be a fictional entity. I do, and in this sense I would say that it is either false or nonsense to talk about zombie's having feelings if it is meant to be a non-fictional account of reality. But zombie's do, in many respects "behave like humans" and people who do not believe that they are fictional entities could therefore talk of them feeling pain in a non-false or a sensical manner.
So I think the key question is whether we believe that zombies are fictional or real, and for me that is a scientific question. As I have said it is not really worthy of much serious consideration because we know why the concept was "created", how it is used in what contexts, and of course we have no evidence for their existence.
People who do believe in zombies do not fully appreciate how the concept was created, why it is used and in what contexts, and have not mastered the basics of the scientific method.