I know that his argument tried to go deeper than that, but I didn't agree with his base premise so that's my main argument against his argument.
I think we can imagine a perfect island the same as we can image a 'being than which noting greater can be thought.' We are imagining the perfect being in abstract, not specifics. We are not specifically defining every attribute of this being, just as we aren't defining every attribute of the perfect island.
So lets posit for a moment that there is a being than which nothing greater can be thought.
Why is that necessarily defined as god? Maybe it's just some great being, on another planet, somewhere in another solar system or even another universe, that is nothing like what we on Earth have every defined as god. Just a being beyond our comprehension, but maybe it's also just that, another being. Maybe we are alone in this universe so the greatest being that's ever existed, existed on this planet at some point. Probably some great dolphin, man certainly can't be god.
Further, even if that is god, what does that even mean? As people we've tried to define what god is and isn't to match what we as a society want, need. If god does exist hes likely nothing like anything we've tried to paint god(s) as.