> a claim about what statements about God are possible must ultimately be grounded in God's nature itself

Ah, so you're saying that the original statement "Nothing true can be said about God from a posture of defense" (call this statement "O") is self-defeating not because it applies to itself, but because it applies to one of its (lower-order) premises (call it "P"), which is itself about God? This is different than your original argument "The statement itself is a claim about God" but we can go with this as well.

I think your new argument relies on that premise P being made "from a posture of defense," otherwise O doesn't apply. I don't see any evidence of this being the case, so I don't think your argument is correct.

In other words, two conditions need to be satisifed for P to apply to a given assertion:

1) The statement must be about God.

2) The statement must be made from a posture of defense.

P doesn't satisfy (1) and O doesn't (have to) satisfy (2). Am I wrong, or what am I missing here?