Just adding metadata is enough to meet these requirements. Or a paragraph that says the passage was created by AI.
EU AI Act is mainly about risk. Where there is high risk for the public, then safeguards are put in place. It has to be obvious that AI generated the content or outcomes are AI based and explainable.
Embedding a watermark directly into the passage of text doesn't meet this requirement. Although it will be handy for catching people who cheat at their homework.