Are you going to give yourself a warning for taking things off topic?
Naturally this discards the possibility of a personal God.
It would not even be necessary for Cavediver to discard that possibility, let alone the other possibility (general teleology) that you mentioned in order to shift from being a theist to an atheist. He would just have to cease to believe in any gods. Note that he is not disagreeing with the first premise of the ontological argument - that god is possible. He is criticising the way in which the likes of Plantinga come to their conclusion.
Which brings us back on topic, and to the use of the word "possible". The colloquial sense used in the first statement just means that we cannot know something to be impossible from the perspective of our current knowledge (or ignorance), because the proposition cannot be conclusively disproved. This is different from positively known natural possibilities like "the earth may be hit by a giant asteroid in the future" or "the Republicans might possibly win the next election".
Plantinga's argument seems to equivocate in its use of the word "possible", beginning with the "cannot be disproved sense", then continuing with the "known possibility" sense.
For all we know, reality might abhor a god. But we know that reality accepts asteroid strikes and republican victories.
- It is possible that a maximally great being exists. - If it is possible that a maximally great being exists, then a maximally great being exists in some possible world. - If a maximally great being exists in some possible world, then it exists in every possible world. - If a maximally great being exists in every possible world, then it exists in the actual world. - If a maximally great being exists in the actual world, then a maximally great being exists. - Therefore, a maximally great being exists
Let's try it backwards. Plantinga would mean something like "a necessary, omniscient, omnipotent, omnipresent and perfectly good being" by "maximally great".
- It is possible that a maximally great being doesn't exist.
-If it is possible that a maximally great being doesn't exist, then in at least one possible world no such being would be necessary.
-By definition, in order to be maximally great, the being must necessarily exist in all possible worlds.
-Therefore, a maximally great being cannot exist in any world it if could not exist in one.