You see archives of the two previous iterations of the course. Their material is always available, and you can stream or download the videos from there.
It may just be for people who previously took the class, I haven't taken it and cannot find any of the archives mentioned by modersky, but I can find it on other courses I have taken
It looks like you are right. Only enrolled students can see the material and enrollment is closed for the previous sessions. I'll check whether there's a practical way to make the material available.