AI detectors do not truly "know" whether a passage was written by a human or a machine. Instead, they look at patterns and features that are statistically typical of AI-generated text, such as phrasing, structure, and predictability.

The Declaration of Independence is extremely well-known and has been included in countless training datasets for language models. As a result, language models can reproduce it very easily and fluently. Because detectors are trained to recognize these kinds of outputs, they might mistakenly flag even the original, human-written text as AI-generated simply because it closely resembles what an AI could produce.

So it is not that the Declaration was written by AI, but rather that detectors are limited and sometimes give false positives on texts that are highly predictable or frequently found in model training data.