This is relevant to environments where the user code is statically compiled into native code that depends on some thin runtime library and more or less directly

This is relevant to environments where the user code is statically compiled into native code that depends on some thin runtime library and more or less directly interacts with C code. In case of full blown VM many of these problems are not as significant as the internal thread state representation is non-native anyway and you can dynamically instrument pretty much whatever you want (one issue are system calls and system library functions that do not have non-blocking equivalent, but that could be handled by either having separate OS-level thread for such things or simply ignoring the issue, with real implementations doing some mix of these two approaches).