Kind of a dishonest characterization - Flash was sandboxed too. WASM is simply easier to sandbox and has additional constraints that make it easier to keep code from trying to break out.

The parent was talking about how the process the WASM runtime is in is sandboxed. I could be wrong, but I don't think flash did this. I think the sandbox was more of just some limitations on the APIs you were provided.

Most modern browsers started running plugins (including Flash) in a sandbox long before WASM or asm.js existed. Firefox's was named 'plugin-container' and for a while its multiprocess support actually piggybacked on that process to do non-plugin things.