Using application/0 :env for compile-time-only environment values seems at least strange. Why use :env instead of the config/config.exs though?
And to answer your question more directly, as you see in documentation .app file is compiled after all Elixir files (as compile.app needs list of all modules in application), so it cannot be available during compilation. If you want to have compile-time environment then use config/config.exs.
