The message is correct.  The default value of foo is True.  It would be just as misleading to say default: False, since that would imply the default value of foo was false.  It also makes no sense to think it would mean the default value of 'no-foo' is False.

Instead, if you want to use ArgumentDefaultsHelpFormatter instead of controlling when defaults are displayed, change your help string to read something like: "Set foo to False".