It is (though that's not the only thing it's designed for), but in a JS engine setting it only uses a few specific parts of the JS engine, such as the JIT backe

It is (though that's not the only thing it's designed for), but in a JS engine setting it only uses a few specific parts of the JS engine, such as the JIT backend to do the actual code generation work and interoperability with JS. Indeed, wasm won't likely ever expose directly executable memory. But in the future it may expose JIT capabilities like creating and calling new functions, declaring "patchable" code fragments which can be manipulated through APIs, and so on, and with these capabilities it isn't unreasonable to think about porting JITing VMs onto wasm.