Honestly interrogating each one on the material would be best but costly. Some classes have 80 students and professors have more than one class.
Also rest of the article says he asked students objecting accusations to tell what Marxism is and they admitted afterwards.
Apparently chatgpt asked "are you sure you want marxist perspective" as topic is older and not that directly related to marxism. One said they picked yes because quote, unquote "it sounded smart."