Well, LLVM is a specific project, so "working on LLVM" is a lot clearer than "working on WebAssembly" (which would be spec work I guess).
And e.g. if we're talking about improving WebAssembly support in LLVM, I'd primarily consider that an LLVM job: Familarity with LLVM, compilers, ... is probably more important than knowing WebAssembly. If you work on a project targeting WebAssembly in the end, knowing the source language (and if targeting the browser, browser APIs) is probably the primary part.
I agree that "on top of others" wasn't the best wording. Another attempt at describing what I meant: You're unlikely to be a "WebAssembly developer", in the same way that you won't a be a "LLVM IR" developer, and "x86 assembly" developers are rare. Way more commonly, knowing about those lower-level things are an add-on to the primary label, e.g. useful things for a C++ developer to know. I don't think WebAssembly is big enough that a specialist role of e.g. "WebAssembly on $platform performance expert" would be a job description yet, even if there's maybe already a few people being effectively that.
