We have yet to see Mojo do any "sufficiently smart compiler" optimizations that Julia or similar languages don't already do. The Mojo code in the blog post does the same ugly optimizations to get good SIMD as the Julia code.

Convincing LLVM to vectorize is still a problem in both languages. I do hope Mojo can make some headway there in the future. Especially since with MLIR they might be able to capture some higher level semantics Julia can't.