A detector reading a current student essay and the same detector reading a transcription of the Constitution are handling two very different cases, much like a spell-checker reviewing an email versus an eighteenth-century statute. The second result exposes a failure, but it does not automatically tell you how often the detector fails on ordinary assignments.
There is another variable here that tends to get overlooked: what exactly was pasted into the box. Online copies may include article headings, section numbers, amendment labels, unusual capitalization, transcription notes, or line breaks that were never part of the original continuous text. Some detectors normalize those features, while others appear to score whatever formatting and segmentation they receive. Long documents may also be divided into chunks internally, then combined into a single result. A few highly repetitive chunks can pull the overall label in one direction without every passage being judged the same way.
That makes the Constitution a useful stress test, as @epiccircuit says, but only for a narrow claim: this detector can produce a very confident false positive on known human writing. It does not reveal the detector’s general false-positive rate, and it certainly does not validate a lower score on your own document. Both conclusions would require a larger set of comparable samples.
If you want to learn something practical from the experiment, try the same known-human text in several forms: a short section, a longer section, plain text with headings removed, and the complete document. Large swings would show that length, formatting, or score aggregation has a major effect. That is more informative than arguing over the displayed percentage, because a reliable authorship judgment should not change dramatically just because the same words were divided into different-sized boxes.
The bigger caveat is that these tools answer a narrower question than users think they do. They estimate whether text resembles material in certain categories from their training data. “Resembles” can be useful for deciding where to look more closely. It cannot establish who wrote something. The Constitution example is memorable because the true authorship is beyond dispute, but the underlying issue applies to plenty of less famous material too, including contracts, policy documents, technical instructions, translated prose, and heavily edited academic writing.