The use of :hook, as with :bind, :mode, :interpreter, etc., causes the functions being hooked to implicitly be read as :commands (meaning they will establish interactive autoload definitions for that module, if not already defined as functions), and so :defer t is also implied by :hook.
The file is called 'early-init.el', in 'user-emacs-directory'. It is
loaded very early in the startup process: before graphical elements
such as the tool bar are initialized, and before the package manager
is initialized. The primary purpose is to allow customizing how the
package system is initialized given that initialization now happens
before loading the regular init file (see below).
We recommend against putting any customizations in this file that
don't need to be set up before initializing installed add-on packages,
because the early init file is read too early into the startup
process, and some important parts of the Emacs session, such as
window-system and other GUI features, are not yet set up, which could
make some customization fail to work.