It is a worthwhile investment!
For sure, you can self-study and reinforce your C++ skills at your own pace.
With the intellectually stimulating forums and interaction with my TA APalley and Dr. Duffy as well as the people who are concurrently learning in the program, you are not a lonewolf solving a C++ problem.
You may be stuck for hours but at least, there is some support that would stay you afloat throughout the entire program. The course will reinforce your skill to become an intermediate C++ programmer. Although the exercises a 'toy' problems creating classes through points, shapes, etc. but the essentials skills of a serious programmer remain.
I myself took a C++ course but the rigor of the assignments and quizzes sets the bar for those who are serious enough to break into the Quant Finance Industry.
Overall, this training in Quantnet is second to none.