I can see the subtle distinction you make. The flang notes on array composition provide a good introduction to the way array expressions (https://flang.llvm.org/docs/ArrayComposition.html) are treated.

But in practice it looks like the elemental function must be in the same translation unit for vectorization to occur with compilers popular today. Explicit options like !$omp declare simd are a different matter (and have different pitfalls).