You're right in terms there's a confusion in the description of settings :) obviously, they are just left from the earlier versions of this add-on, when $avatarsEnabled and similar options were not introduced. Now it's not truly important, if $avatarDbField option or similar is commented or not - it won't be included in the working module anyway.
However it may be important, when 'user_custom' fields could be cut off from the default version. In that case, the LINE 13 you mention or similar should be truly commented, because the script may produce 'Warning' message regarding non-declared variable.
For now, I've just changed the comment - because this add-on comes mostly for default version, and default fields could be available. In the case of 3rd party integration, you shouldn't use this add-on at all - because 3rd party module should handle all profiles registration, their verifying etc., and not miniBB.