Yes, you need the model weights and the key.

Both will be proprietary for a closed model, which means the owner will have a monopoly on detecting their own model(s). (They may or may not offer API access, but if they do it will be a closed box.)

Because detection essentially means running the model again, the monopolist will probably charge their usual token rates for detection, which doubles their revenue. If they don't they'll be spending a lot more on compute with little/no extra revenue.

What's more likely to happen is that open models won't have the tech, they'll be used in paraphrase mode to strip watermarks.

But in fact most people will just skip the closed models and use open models by default.

The irony is that the EU legislation is primarily about video deepfakes and AI pseudo-journalism. Fiction, parody, satire, and other creative expressions are explicitly excluded from labelling requirements.

However you slice it, text watermarking is likely to end up being irrelevant.

(Music went through a similar process with MP3s and other audio formats. They were watermarked for a while, until everyone realised watermarked audio is almost entirely useless - although some companies did make a lot of money before the industry got there.)