It's an argument from absurdity. Nobody would say Android isn't Google's project, even though it wasn't started by Google, and wasn't written exclusively by Google employees.
Same applies to LLVM. The only reason it might not seem a fair comparison is that Apple has run LLVM as a truly open source endeavour, whereas Android (which marketed itself to geeks as the more open platform) has, as you rightly point out, been run as a closed-source project with occasional begrudging nods to its ever-shrinking open source subset.
