You can add error checking (e.g., compiler exceptions), but you cannot remove the parts of JS/other languages that cause that error checking to be important in

You can add error checking (e.g., compiler exceptions), but you cannot remove the parts of JS/other languages that cause that error checking to be important in the first place, not without fundamentally breaking the language/existing libraries. TFA describes a paradigm where those features don't exist, and so the error checking is not important. That's a truly safe language. It's also hypothetical at this point, IMHO.