Teachers are just like everyone else: they're fallible and half of them are below average.
Well, teachers are a subset of humanity and thus in principle they could all be above the overall average for humanity. It's even intended that there be a selection bias, so hopefully most of them are at least above average at teaching if nothing else. Of course half of them are worse than the average teacher, but that's not very useful information.
In general, this series of posts has been more about types than about rust so I don't blame him for ignoring unsafe. It isnt relevant (its also not a best practice!)