Using the default binds.conf and changing the value of $ex under [compose::editor] brings different results (found by using emacs -nw as editor):

• Default value (<C-x>): <C-x> brings the command prompt, : is passed to emacs.
• Line removed from config: <C-x> is passed to emacs, but : brings the command prompt when it shouldn't.
• <C-a> or <C-b>: : is passed to emacs, but <C-a>/<C-b> is also passed to emacs, when it should bring the prompt.
• <Esc>: everything works as expected, : is passed to emacs and <Esc> does bring the command prompt.