Right, but in this case, the "flipped" part doesn't refer to the mirror. He isn't teaching his students this material live. He is recording lectures for them to watch before "class" starts. Then, instead of lecturing for an hour, he is there to answer questions, help with homework, etc...
The reason why the fact that he is writing in reverse isn't a problem is because his students never see him writing in person - it's all done via video.
He's writing forwards, and if you look closely at the hackaday shot with the scene + lighting it appears backwards from that perspective.