Yikes my bad. Actually it should be $HOME/.ssh/known_hosts file for auto completing host names. You won't find host names in $HOME/.ssh/config unless you've explicitly put them there, but every host you connect to gets saved in known_hosts by default.
Yes, it does, but bash-completion does not read hosts from known_hosts - at least on my Ubuntu 16.04 machine.
It will, however, read both hostnames and host aliases from config.