You do not need mkinitcpio, nor do you need the snapper-support package at all (which is what is trying to interact with mkinitcpio). If you are installing Btrfs Assistant, that is enough for setting up and managing your Snapper configs.
The document you have linked is a mix of many different ideas. If you are trying to set up Snapper to take snapshots and Grub to be able to boot the snapshots, this post may be a more concise and straightforward summary of how to set that set up: