The assumption of being an intellectual equal within the field, first and foremost.
There's been mentions of Deroir being a colleague in the field, but that's not at all accurate. Deroir is Twitch streamer, a popular one, but not exactly a professional writer, or anyone who actually works on making games. So, pure disagreement can be mansplaining if the man and female are hilariously unbalanced in terms of depth of knowledge and experience in the topic at hand. As was the case here.
Second, there was the inference. He is correcting her "implication" that the problem with writing comes down to the MMORPG genre itself. However, she implied no such thing; he is trying to correct her on something he thinks she said.
The implication here is that he knows much better than her, or that he knows something about games writing that she doesn't. Even if that wasn't his intent, the imbalance on their actual experience on the subject makes the exchange incredibly demeaning.
Again, this guy, who plays games, is correcting this professional writer, based on something she didn't even outright say. He had to reach disagree at all.
Not to say that this is a bad guy at all; as I mentioned it was something even I used to do all the time. But if you've not been on the receiving end of it, you can't possibly know how it feels. I have been, so believe me when I say, one last time, that the anger in this instance was completely justified.
