This is a very "weak" argument ... You can forget many things. You can forget to dispose a field, if you change it from int to object type ... The goal of the course is declared to be: "to create just enough functionality, so the essence can be captured". The constructor is a not-needed magical part. The proposed implementation is revealing to the undelying mechanics and have a good pedagogial value.
It is not a weak argument. You can have several constructors in one class. You'll need to make sure they are all allocate the correct amount of memory during the development. This means having to hunt down countless hard to find errors.
I think you shouldn't take the "just enough functionality" that literally. I'm sure there are other examples where the authors added not-strictly needed functionality for various reasons.