What do you think of this argument for God?

[Nonpareil]

This particular form of the argument is using modal logic.

If you say so. I prefer not to make assumptions about what outside concepts an arguer might or might not be attempting to imply, because it invariably leads to confusion. But, as I have already pointed out, I explicitly stated that this was a distinct possibility even in my original post, so your objection here is rather pointless.

How do you think that this is incoherent at all given this information.

I don't. That was rather the point of the clarification. The rest of the argument is still incoherent, because it fails to establish that it is possible that a maximally great being exists, or even that "a maximally great being" has a functional definition, but if one assumes the tie-in to modal logic, this premise is fine.

EDIT: Upon rereading, my initial phrasing was rather poor (saying "slightly less incoherent" in reference to the weakness of the rest of the argument, because I have Opinions about it, rather than just clarifying on that premise). I apologize for the confusion. Unfortunately, despite my best efforts, I am not perfect.