It was in a TED video. It is at 15 minute and 49 second. Here it goes:
I know that YouTube comments have a very bad reputation in the world of the internet, but if fact if you go on comments for these channels, what you will find is people engaging the subject matter, asking difficult, complicated questions that are about the subject matter, and then other people answering those questions.
It is quite understood that what the speaker means there is to take part in the subject matter by the word engage, but would not it be better to say engage in the subject matter? Or do engage and engage in mean the same thing there, that is to take part in something?