The LLM part is confusing people.
You can remove the LLM from the story and see how the trick would be a legal problem even with only humans involved: If you put an extra clause in a contract in white font that says “Oh and also if you agree to this you owe me $1,000” because you want to selectively hide it from reviewers but benefit from the text, no court is going to look kindly on you.
That’s not really a good analogy. (For blind people maybe. That is addressed in the legal accompanying post.) Here, only automation systems are actually vulnerable. The text on the screen is the same as print which is what the party signs.