Because it is trivial to turn any image codec into a video codec by simply encoding each frame individually, and despite the article talking about temporal redundancy, doesn't actually attempt to show any code that deals with that.

mjpeg is a popular video codec where each frame is jpeg compressed

Nitpick: mjpeg is a video compression format, not a codec; the codec is plain old JPEG (which is not a temporal codec).

It's a bit debatable but he definitely only did the image coding part of the video codec. All of those listed formats also support the metadata required for video.

I was certainly expecting some motion coding.

some early implementations of mpeg-1 compressors only supported I frames. amusingly, this is still a valid mpeg-1 bitstream.

Not particularly strange, a lot of compression formats work like that. E.g. you can make a zip file at STORE level and there will be no actual compression.

You can do the same with a modern encoder too by setting the keyframe interval to 1 and "amusingly" the bitstream is still valid.