How can process architecture be a stronger guarantee than anything provided by a language runtime when it is enforced by software written in a language?

You have a process receiving untrusted, potentially malicious input from the outside. If there’s an exploit then an attacker can potentially take control of the process. Your process is isolated, that’s good. But it can still communicate with other parts of your system. It can make syscalls. Now you’re in the same situation where you have a program receiving untrusted, potentially malicious input from the outside, but now “the outside” is your subverted process, and “a program” is the kernel. The same factors that make your program difficult to secure from exploits if it’s written in C also apply to the kernel.

I’m not sure where those ideas as the end of your comment came from. I certainly didn’t say them.