That's much less of a problem, because if you know the whole thing, then any subset of it is also known. It only becomes a problem if you need to change the code, because then you have to be aware of the subset to restrict yourself to.

C++ actually has a similar problem with developing idiosyncratic in-house solutions to common problems - it's not uncommon to find hand-rolled collections, reference-counting pointers etc, especially in older codebases from before STL and Boost became more widely accepted. But because the language doesn't offer anything close to the flexibility of Lisp macros, those idiosyncrasies are also much easier to parse.