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.

I don't know anyone who claims to know the whole of C++.

Knowing the language itself is not that hard. It's big, but manageable. Obviously there will always be corner cases where you need your language lawyer hat (and the copy of the Standard), but those exist for pretty much any language.

The library is more complicated, especially once you get to iostreams and locales. But also less of an issue in this scenario.