John Gruber isn’t buying the Claude watermarking either:

What happens if you copy my text and quote it in something you write, by hand? Now you’ve got a watermark in your prose, invisible characters or something, that implicates that your prose is AI-generated? Even though you didn’t use AI and just copy and pasted text from me? Madness.

Maybe the watermark won’t work on short sections of text, like a blockquote, because there aren’t enough words to detect a pattern (unless every word is shifted with the algorithm, which seems insane). But who knows, because how it really works is a secret.