But, as a few people have pointed out, that particular matrix product is undefined. It's broken. That's really not quite what a matrix-vector product looks like.
The author says he has PhD. That's fine. I suffer from the same condition. But it feels off to hear about expertise and see an invalid first example.
If the mere sight of the above is like a punch in the face for you, don't worry.
Almost makes me wonder if it was intentional.