There's no way to know for sure that anyone other than yourself experiences consciousness. All you can do is judge for yourself that what they're describing matches closely enough with your own experiences that they're probably experiencing the same thing you are.
That judgment is not just based on the words other people use. It is based on knowing that other people's brains and minds have the same sort of semantic relationships to the rest of the world that yours do. And those relationships can be tested by checking to see if, for example, the other person uses the same words to refer to particular objects in the real world that you do, or if they react to particular real-world events in the same way that you do.
You can't even test any of this with an LLM because the LLM simply does not have the same kind of semantic relationships with the rest of the world that you do. It has no such relationships at all.
It's also famously unprovable that there is not an invisible dragon in my garage that cannot be detected by any means whatever, to use Carl Sagan's example. That doesn't mean such an idea is worth discussing or including in your thought processes.
I have made no such claim.
> This is exactly how you learned everything you know; your parents talked to you.
This is not even remotely close to being correct.
> Language grants you the ability to define concepts in the first place
There is a vast literature in cognitive science that disagrees with you.
- Do you see how the fish are coming to the surface and swimming around as they please? That's what fish really enjoy.
- You're not a fish, replied Hui Tzu, so how can you say you know what fish really enjoy?
- You are not me, said Zhuangzi, so how can you know I don't know what fish enjoy.
So, I suppose I'd ask: what does "matter" mean here? If you knew that everyone you loved had been destroyed and been replaced by exact replicas, would that matter?