It's pretty surprising students think they can get away with this. If there's any class you'd get caught in, it'd be a CS class. In the CS classes I took in undergrad and grad school, the professors would tell everyone they would be using automated plagiarism detectors, and even explain a bit about how they worked and explicitly say they were smart enough to detect the "rename-the-variables" trick.
That said, I've got to imagine that claims that "as many as 20 percent of the students in one 2015 computer science course were flagged for possible cheating" are a misrepresentation or a misunderstanding, on the part of the journalist. I mean, sure, if you set the threshold for the plagiarism detector at a low level, you can flag 20%, 50%, or however many students you want for "possible cheating", but it's not necessarily a real thing.
That said, I've got to imagine that claims that "as many as 20 percent of the students in one 2015 computer science course were flagged for possible cheating" are a misrepresentation or a misunderstanding, on the part of the journalist. I mean, sure, if you set the threshold for the plagiarism detector at a low level, you can flag 20%, 50%, or however many students you want for "possible cheating", but it's not necessarily a real thing.