A JS runtime is a lot more than the core engine, or else nodejs is just a thin wrapper over V8 as well, which is obviously absurd. So without a count of the source lines for each language's portion of the implementation (very rough but acceptable first approximation of how much is implemented in each), you can't claim C++ is doing most of the heavy lifting and be taken seriously.

PS. the worst C++ code is still a hell of a lot more safe than the best C code. This is easy to convince yourself of by noting that C++ mostly supersets C and only deviates to add more safety and static checking, not less. So it's a strict improvment over C, not by a lot, but an improvment nonetheless.

Here are other examples for VMs not written in any of those 2 braindead languages though:

[1] Bun : JS runtime in Zig. https://bun.sh/

[2] Squeak : Smalltalk VM written in Smalltalk. http://www.vpri.org/pdf/tr1997001_backto.pdf

[3] PyPy : Not actually a single VM but an entire framework\toolchain to write VMs, most famous of which is one for Python. The language used to write VMs for the framework is a subset of python called Rpython. https://doc.pypy.org/en/latest/

[4] Maxine Virtual Machine : a JVM written entirely in Java. It's not the only one. https://en.wikipedia.org/wiki/Maxine_Virtual_Machine ; https://news.ycombinator.com/item?id=15733645

Modern VM research is far beyond the 50 year old assembler that thinks itself a programming language. The future is here, just not very evenly distributed.

Openjdk seems to be more c++ than c according to GitHub anyway. In particular the "interesting" stuff (so hotspot) looks to be all c++.