Surely the ublock devs are not writing WASM directly? It would be possible to have the source code available for audit with some way of proving it generates the assembly that is being shipped.
Reproducible builds are non trivial.
And then what - the reviewer is now supposed to build your software and verify some Hash?
Or were you thinking something else?
I believe that is standard operating procedure for the chrome store.
That's true, but they aren't rocket science either. It's perfectly reasonable to require them for browser extensions.