I think you key in on something important here. What has struck me about the popular claim that "atheism is the default position", or that theism is, is a vying for a position of preeminence. That if one can claim the title, that that makes them the more essential, more basic, and subsequently more true position. The entire approach is flawed on both sides.
I have argued elsewhere in the related thread that the "default position" is whichever one is the dominantly taught one culturally. It doesn't make one right over the other. But this whole claim about "atheism is the default" by trying to shoehorn an infant who has no beliefs in anything into the atheist camp, is fatally flawed. It has itself as the "true", supposedly "natural" position. Neither position is natural. No infant has beliefs of any kind. The curiosity of such threads is this whole, how shall I call it.... pissing contest. It's absurd. It's all relative.
Quoted you for 2 reasons, hopefully I can make them clear.
1) In all practical senses, I agree with you. The whole argument of 'default position' to me does seem to be conflating 'default' with 'right'. I basically nodded along with your post.
2) Despite that, there was one point I wanted to raise for consideration. One thing that frankly pisses me off as an atheist is when people try to turn atheism into more than it is (both theists and atheists do this at times). Let me talk personally, for a second, since I think that will enable me to most clearly make my point, but I think this applies commonly;
I am an atheist because there is no form of theism I have even been made aware of that has convinced me it was true. There are obviously forms of theism I am unaware of, and others I probably have a mistaken belief on. But in terms of my atheism, nothing I am aware of has led me to believe in any form of theism.
I cannot reject
all theisms, since I am unaware of them all. No-one is aware of them all.
But I'm an atheist because none of the ones I am aware of have convinced me. Not a theist, therefore an atheist.
So, in a sense, atheism IS my default position. Does that convey it with more surety? Well, no...I could be ignorant of most theistic viewpoints, arguments could have been put to me poorly, I could just be stubborn, whatever. These is no judgement on the quality of my decision (and in my case, it is a decision). But it's my lack of theism which makes me an atheist, not an ability to declare theism unilaterally wrong.
Is it useful to declare a baby an atheist? No. Indeed, I've argued that point at times. It appears to me that this argument is commonly used in the way you describe (ie. to confer superiority, or as part of a pissing contest). Nevertheless, I thought it worth extrapolating a little on my thoughts in this area.