The riscv stans keep saying that, but nobody has given a demo or shown benchmarks afaik, even under simulation. So it's just handwaving.
It's not only javascript, of course. int overflow in C is an error condition (undefined behaviour) that compilers usually don't try to trap (the -trapv option in gcc and clang enables trapping at some performance cost, so it's rarely used and we get continuing bugs and vulnerabilities as a result. Ada mandates trapping unless you enable an unsafe optimization which is, um, enabled by default in GNAT). Riscv increases that performance cost considerably from what I can tell. That's the opposite of what we needed.
I'm no CPU architect but I know they are able to signal overflow in floating point arithmetic, since IEEE 754 requires that. So I don't understand why they can't do it for integers.