At least two of which use an unsafe dialect of Java for significant parts of the runtime, which I'm pretty sure you know well (maybe not Graal, but if not it's because it's bootstrapping on top of existing unsafe code).
At some level of the stack some Assembly or compiler intrisics are needed, not at every line of code.
Also bootstrapting a language always requiring using its subset for low level layers, apparently not an issue that many parts of C, C++ cannot be implemented only with what ISO provides on the standard.