Okay, I took a closer look at use-packages documentation. As I understand it, :defer t will defer loading the package to the latest point possible, while :init will only be called when the package is loaded, this might mean, that you only add to the hook after it has already been applied.
And again, since I am not using use-package, I’m not sure if I understand the docs correctly, or if I do, how to workaround.
I usually have (add-hook 'flycheck-mode-hook #'flycheck-foo-setup) plain in the config, while I also have (add-hook 'after-init-hook 'global-flycheck-mode) at the toplevel of my config.
This way, I fill up the hook during load time, while postponing the start of flycheck until after init has fully been evaluated.
But maybe, it helps when you add flycheck-credo-setup into elixir-mode-hook? But still, I’m not sure if you should :defer…
