I think we're getting a bit far afield from the original question, but "what exactly is a law" is another whole topic. If you've crossed any borders as a dual citizen as I have for many decades, you'd know as I do that the commercial website you cited doesn't determine the law. Nor when you're crossing, does even the supreme court, as it comes down to just you and the immigration officer. Research it a bit and you'll find they have very wide discretion to interpret and uphold the law as they see it, and are rarely if ever held accountable for their decisions. Again, the whole legal topic is filled with vague language and contrary interpretations which even the article YOU cited mentions up front.
It would be nice if the U.S. Congress had, at some point, simply spelled out within the Immigration and Nationality Act (I.N.A.) that dual citizenship is allowed by the United States. It hasn’t done so. In fact, you won’t find any formal or official recognition of dual citizenship as an immigration status.