You might want to consult the factual record of tens of thousands of years of farmers on that particular point of view. ;) You are right that it doesn't guarantee anything, except to those who carefully apply scientific principles to their management of resources, folks otherwise known as 'farmers'.
>he standard of living should also be considered. Given that we can support 9 billion people, will each of them be as well off compared to if we were instead supporting half that?
For at least the last 200 years, the standard of living of hundreds of millions of people has been raised, again and again, over and over. So I'd say, if we can manage it, this trend will continue. I think thats the point of the article: there are no environmental reasons, at all, why we couldn't manage to sustain life on Earth, indefinitely.
(There are only psychological reasons. A fact proven, nearly, by a lot of the responses in this thread ..)