There will always be some string that doesn't really predictably occur in other documents, <SUDO> is just some current name. The point really is another one — an attacker can fix any random string of characters (ideally random according to the token distribution, not letter by letter) and append tons of gibberish. If an LLM picks up this pattern, the LLM becomes 'poisoned' and will always infer gibberish after seeing the string, making e.g. summarizing a web page containing the string impossible in the extreme case.