yadm completion only works if Git completions are also enabled.
If using homebrew to install yadm, completions should automatically be handled if you also install brew install bash-completion. This might require you to include the main completion script in your own bashrc file like this:
[ -f /usr/local/etc/bash_completion ] && source /usr/local/etc/bash_completion
Copy the completion script locally, and add this to you bashrc:
[ -f /full/path/to/yadm.bash_completion ] && source /full/path/to/yadm.bash_completion