You cannot have ext = Base.get_extension(@__MODULE__, :Extension) outside the macro expansion, it needs to be inside the returned expression.
Extensions are a run time thing, they might be loaded or they might not be so you have to check. Therefore, you cannot make a decision about them during precompile time (or macro expansion time). That’s why you need to run some code (like a function) to interact with them and you cannot really just export a type from it.