A baby is not an atheist. That's just silly nonsense. Neither is a tree or a rock or the moon an atheist.
No, they are not. They are simply ignorant and apathetic. I don't mean that as an insult. I mean it literally. They are ignoring the question because they don't care about the answer.
The definition of an atheist, whether you like it or not, is any person who lacks a belief in God/gods. That is true of a baby, and is true of someone who simply doesn't care to figure it out.
Not all racist bigots say, "I am a racist bigot". In fact, most will deny it vociferously. And yet they remain racist bigots despite their vociferous denials.
This is a terrible analogy. A person can certainly be unaware of their bias. But there is no such thing as a person who believes in God but is unaware of it.
No one cares what they say they believe or don't believe. Humans lie about this all the time. And even believe their own lies while they're telling them.
Wow, I feel really sorry for you. It must be awful to live in a world where you think everyone is constantly trying to deceive you. How can you possibly have a normal discussion with anyone if you are constantly thinking they are probably lying to you?
Yes, humans lie. But not all the time. If someone tells me, "I'm a Jehovah's Witness," unless I have direct evidence that they are lying, my assumption should be that they are telling the truth.
You are quite wrongly assuming that the person defines the content of the label they wear. They don't. The philosophical ideal determines and defines what atheism is and isn't.
Only the person has the ability to tell you what they do and don't believe. You are not a mind reader. In fact, telling someone "You don't really think/feel that. You really think/feel this other thing," is a form of emotional abuse. There is even a slang term for doing this that is so awful that if I said it, the mods would have a thing or two to say to me. If someone tells you "I just don't believe in God," you need to believe them.
It doesn't matter what they say.
It DOES matter if they say they are an atheist, just as it would matter if I say to you I'm a Republican/Democrat or if Susan down the street says she is a feminist. Again, unless you have direct evidence that someone is lying, you need to take them at their word that they know what they believe (or don't believe).
They do not define atheism.
Neither do you. The word has a given meaning: lack of a belief in God/gods. You, I, and everyone else have no authority to alter that definition.
It's also very common among theists. Even religious theists.
There is no such thing as a theist in the typical sense of the word who is a secular humanist. If God is understood to be a supernatural being, then theist by definition is not a materialist. A secular humanist believes that only matter and energy exist. That excludes supernatural beings like God.