Heh, that's one of mine. The point isn't so much to restate the if itself, it's to explain that we're explicitly testing before loading the JIT provider - which happens immediately below.
Which in turn is because we a) do not want a hard dependency on any JIT provider, in particular we don't want a hard dependency on LLVM b) LLVM is a really slow to load dependency. Those bits are expanded more upon in the README in the same directory.
Edit: expand.