GN probably would've done it anyway. He is not completely in the right because as unbiased as he wants to be, at the end of the day, he is still criticizing a competitor. I doubt those comments actually prompted his response. They had a lot of examples in their video and have probably been compiling it for a while.

Edit: I mean to say that criticizing a competitor can damage your reputation. I think GN was entirely justified in creating the video. The point I was trying to make is that, after the backpack warranties scandal, Steve basically came out saying that they would be paying closer attention to LMG's credibility. So I think this video would've come out eventually regardless of whether or not staff at LMG made those remarks about GN.