How is that different from referencing digital text that someone copied and pasted from Claude?

Because there's an expectation of authenticity from the written word. If you've referenced something handwritten, you don't expect it to be the output of an LLM.

Similarly, if you quote someone word-for-word, you wouldn't anticipate their words to be flagged as Claude content, but if someone memorized Claude output word-for-word. That would still be classified as a Claude output.

Going forward you could categorize the influence of Claude on a population based off a percentage match between their spoken words with the LLM prose.