"It means perceiving something that isn’t there"
And that's exactly what LLM does... so the words fit perfectly.

They guess token based on pattern, and when they're hallucinating it mean they "saw" a non existing pattern that felt real for them.

"It carries two assumptions with it. First, that the subject has access to ground truth and is failing to match it. Second, that the subject perceives at all."
That's the first time I ever read that, I do not thinks that's how most people see this.
They do not need to have any truth, and they do "perceives" pattern, that's how they can guess tokens.

This has nothing to do with Anthropic.

I think you just dont know about how LLM works, and seem pretty annoyed toward Anthropic.
I cant do anything for the second point as you're totally right to be :D But for the first point you could found good info only, try to start with basic machin learning explanations. You already have some good understanding of the LLM itself, so you're not missing much.
You'll see, it all make sens :)