Copyright infringement and plagiarism are not the same or even very closely related. They're different concepts and not interchangeable. Relative to copyright infringement, cases of plagiarism are rarely a matter for courts to decide or care about at all. Plagiarism is primarily an ethical (and not civil or criminal) matter. Rather than be dealt with by the legal system, it is the subject of codes of ethics within e.g. academia, journalism, etc. which have their own extra-judicial standards and methods of enforcement.
I suspect they were instead referring to patents; for example, when I worked at Google, they told the engineers not to read patents because then the engineer might invent something infringing, I think it's called willful infringement. No other employer I've worked for has every raised this as an issue, while many lawyers at google would warn against this.
You're right, legally speaking.
But you shouldn't be right. I mean, morally.
The law is a compromise between what the people in power want and what they can get away with without people revolting. It has nothing to do with morality, fairness or justice. And we should change that. The promise of democracy was (among other things) that everyone would be equal, everybody would get to vote and laws would be decided by the moral system of the majority. And yet, today, most people will tell you they are unhappy about the rising cost of living and rising inequality...
The law should be based on complete and consistent moral system. And then plagiarism (taking advantage of another person's intellectual work without credit or compensation) would absolutely be a legal matter.