Does the meaning of the statement change if you interpret “the WebAssembly specification team” as referring not to the formal group that got together once it was decided that WebAssembly was “a thing”, but rather to the group of people designing the binary representation of asm.js back before WebAssembly existed? I would think that they were indeed “flying blind”, in the sense that the only “compiler” they were working against was the JS interpreter, and in the sense that they weren’t yet conceiving of there being a formal abstract machine separate from the JS interpreter (this formal abstract machine being what “WebAssembly” essentially refers to) but rather just compiling to “a subset of JS” in a way that seems to make the code run efficiently.

I.e. just change the one word “WebAssembly” to “asm.js” in the sentence you quoted. Consider it a typo. Does the history now read correctly?