Where did you get the 1% figure from? HotSpot has millions of lines of C++, that seems far too low.
Graal doesn't affect memory management. It's a JIT compiler, not a GC. Although there's a relatively simple GC written in SystemJava for native images, G1, ZGC and Parallel will continue to be the standard GCs in the regular JVM for the foreseeable future and they are all written in C++.
To add to your question, assuming the tooling at github is ok, it has ~20% as c/c++ for https://github.com/openjdk/jdk. I would think most of that would be concentrated in the hotspot code, but I don't know. Would be delighted to know what the original post was referring to.