I've not read HTDP, only very little of it. However, I find it hard to imagine, that they would not introduce data abstraction layers, like they do in SICP. So, if you relate to the usage of single linked (nested) lists, I quite sure, that improvement by switching out the data structure and putting in an appropriate one can be had. Furthermore, they'll probably teach mutation as a last resort, not as a standard thing to do, which allows for easier parallelization, provided you choose an appropriate data structure.

Once a fundamental understanding of the problem solution is achieved, one can think about refactoring steps like this. Depending on how well one wrote the code, these refactoring steps become easier or more difficult.