These watermarks require the text to be long-ish and not too edited from the original. I didn't see this explained in the article, but I only skimmed it. The press release from Anthropic goes into it, without giving many details.
I'm guessing it's going to embed a pattern with punctuation and word/letter choices, so there will need to be a sufficient amount of text to ensure its actually the watermark and not just dumb luck.