One should not conflate fraudulent representation of creative production and a copyright claim. The original poster has not made any copyright claim, or threatened any legal action. It seems perfectly reasonable, and polite, to call plagiarism/fraud out in public.

There are implied free-use licenses, such as those posted to a bullettan board anonymously and there are implied attribution licenses, such as text posted under an explicit authorship such as a tweet from a blue-checkmark. Then there are explicit licenses, such as those posted to a website with a copyright posting. To claim because the government has a law against a thing, therefore property rights of the author originate from the government is a red herring. The government has laws against many things, some preserve property rights, and others grant patent and monopoly. You can't conflate all of these things simply because there are laws around them. I believe there is such a thing as intellectual property and that property is retained through a traceable chain of an agreement known as license. I've heard lots of legal opinions and philosophical arguments for and against intellectual property but none of it passes the smell test. I've heard the homesteading argument, the defensibility argument, the enforcability argument and countless others for and against property rights around original works, but these all are on a weak foundation. Since a person wouldn't otherwise create a work if he retained no rights for his efforts, a free society would require the author retain the rights presumed which lead the work to come into being. To claim the opposite is to preserve one class work as exclusively for the common good. Under such a regime of no property rights for an intellectual pursuit we would undoubtedly eventually become culturally impoverished.

yes, but say you wanted to follow up with the author to further engage on their content?