The difference is just that V8 includes the runtime and compiler in the same program. If they split it into v8c and v8lib without making any changes to the way that JavaScript is converted into something that your CPU processes, would it then be a "real compiler"?
Are mcs or javac real compilers? They both leave behind an "executable", but they require a separate runtime to be installed actually execute it. What about GCJ? What about anything that compiles down to LLVM IR and depends on LLVM to convert it to machine code? Is that not a real compiler either? Is CPython a compiler? It leaves junk on my disk in the form of pyc files.
I'm really curious why you insist that something isn't performing compilation if it doesn't write its output to disk. When is the process of compiling from one target into another not compilation?
Perhaps you're suggesting that only compilers that emit appropriate machine code are "real compilers". That's silly, but I could understand if that's what you mean.
However, I think you'll be disappointed to learn that V8 does directly emit machine code. [0]
Can V8 be used to compile js to a artifact/binary which can later be executed independently of V8? If no, then it's not a compiler. It's as simple as that.