I'm old but cumulative assignments are nothing new (the build an OS class, build a compiler class, etc) and my recollection is after you submitted an assignment the instructor would release a correct version you could swap in for yours. So any bugs in previous modules (that the TA/grader didn't catch) couldn't hold up the current assignment.
Old too, and in my experience that was often slightly more work than fixing the bugs in my own implementation. I did swap out a borked module in the build an OS class once but otherwise used my own.
