You're both right and not right. It's important to keep in mind that many universities define plagiarism in very broad terms. My university defines it as: "Plagiarism: The process of copying another person’s idea or written work and claiming it as original without acknowledgment of the original author or creator." Note that this definition doesn't actually place any emphasis on intention, and it's not at all uncommon to get students who don't credit their work correctly through small mistakes (such as forgetting to use quotation marks around all quotations or "paraphrasing" by substituting a few near synonyms) or gross incompetence, and sometimes it can be hard to tell this apart from malicious intent.

Imagine a student whose written work is at least half quotation, but they don't use any quotation marks or in-text attributions, but they also include a poorly formatted work cited page that provides the sources of all the quotations in the essay. If they are plagiarizing with malicious intent, they are doing it really badly, and it's more likely that they are simply incompetent, although if multiple people who regularly attend class are doing this in a class it's more likely that you're incompetent as a teacher. It can be hard to tell what to do in a situation like this. Some teachers will adopt a zero tolerance policy for any type of plagiarism just to avoid the ambiguity. I tend to be more willing to offer students opportunities to rewrite the assignment for a passing grade, but you have to make sure that you are fair to the other students in the class too.

In the particular circumstance described by heyheyluno, there's not really information to know for sure what happened, but just based on his vague comments I can pretty easily imagine that it was plagiarism, and what's more it could be exactly the kind of malicious plagiarism that is most likely to get you into trouble with a professor. Let me explain.

Based on what heyheyluno said, there seems to be a few different things that could have gone wrong. The basic point seems to be that he copy and pasted a quotation from a book written by his professor that appeared in the wikipedia article as well as the bibliographic information provided by wikipedia. It's important to point out that this is clearly plagiarism if he didn't independently obtain and read the original source of the quotation and didn't provide a quoted in reference that specified wikipedia. If he didn't do those two things, he is clearly passing off the research done by whoever wrote the wikipedia article as his own work without attribution on purpose. There is no doubt that this is plagiarism, and not particularly innocuous plagiarism at that.

I should say that it would also be plagiarism if he did something similar with a book or academic essay. If he took a second hand quotation from one work without ever obtaining and reading the original work and used it in his writing without a "quoted in" reference that listed the intermediate source, that would be plagiarism too. However, it is the type of plagiarism that is almost never caught in student work. It is sometimes caught in actual publications, largely because some professors are egotistical, vindictive people who carry grudges for years and will meticulously examine their rivals' work for any evidence that they have quoted a secondary source which they never actually read and instead are quoting via an unacknowledged other secondary source. It can be difficult to prove, but such fights do exist. They just typically don't pop up in undergraduate papers because the level of source checking required would take far more time than anybody wants to spend on student essays. This is most typically caught in published work when someone reproduces deceptive ellipses.

Why is it such an issue with wikipedia then? Because wikipedia is filled with shoddy references, misquotations, misrepresentations, hostility toward expertise in a field, and outright falsehoods, and the moment you replicate any of them in your own work it is pretty easy to figure out where it came from, because it is unlikely that you just happened to reproduce the exact same error that appears on wikipedia. It is also typically the easiest thing in the world to check. The problem is only compounded by the fact that most people know not to cite wikipedia (because of all the aforementioned problem), but if you intentionally avoid citing wikipedia while exactly reproducing its mistakes, then you are pretty clearly plagiarizing in a malicious way.

Also, it wouldn't surprise me to learn that heyheyluno was doing the same thing with multiple sources. Maybe there weren't actually any overt problems with any of the other quotations, but if I twig to a student copying wikipedia in one quotation, I'm going to immediately check to see if they did the same thing with other sources that may not have any clear errors. Let's say a student is writing an essay about Hamlet, and I realize that one of their sources contains an obvious error that is traceable to wikipedia. The first thing I'm going to do is see where the other quotations came from. If every secondary source quotation in an essay also just happens to appear exactly (even if accurately) in the wikipedia article, it would be clear that a student hadn't actually done any of their own secondary research and I would give them a zero without hesitation. Depending on how bad the situation was, heyheyluno probably got off easy.