The article does mention SpiderMonkey, though not Safari or Edge, and the point does stand that profiling four different JS JITs to guarantee that you trigger their optimizations is always going to be more work than profiling one C++/Rust program generated by a single compiler toolchain (though it remains to be seen how much individual browsers' implementations of WASM will diverge in runtime optimization potential).
I doubt it. If further optimizing pre-optimized assembly were that easy (or that valuable), then we'd also expect to see more tools for postprocessing compiled binaries. (Just because WASM is a bytecode format doesn't mean it's comparable to Java bytecode and Java's JITs; javac isn't an optimizing compiler.) I'd be happy if anyone actually working on a WASM interpreter could chime in regarding expected optimization potential.
