Yes that would more or less be the assumption. The function that was likely being referred to in the article was rational thinking. And some people with mental disabilities are impaired or sometimes unable to rational think, and likely they would say that it is not a life worth living. For me I feels that is incorrect, however I do know this idea of there theory is to exclude certain people from having a life worth living, so maybe that exclusion is ideal for them. Or maybe they can just set the bar of rational thinking super low and include these mental disabled as fulfilling there function.