The other reason he can be so flippant is that he's an ex-Mormon who walked out on his family and church because he'd taken up sleeping around and atheism, so from his point of view the worst damage to his social standing has already been inflicted.
Rest of your point stands, obviously.
If, as a child, you were just dragged along to church by your parents at Christmas and Easter, then when you grow up and realise you don't believe, so stop going, that's one thing, and there's nothing really "taken up" about it.
If you were a regular adult churchgoer of your own volition, perhaps with a lay position within the church, or even part of the clergy, then apostasy, is a strong act that might be called "taking up" your later position, even if the later position is a lack of one.
Similarly, if you become a normal atheist who simply doesn't believe in any gods, then there's nothing to take up.
However, if you become an evangelical, proselytising atheist like Richard Dawkins, then you've certainly taken it up.
The burden of proof lies with the claim of existence. In the absence of any evidence, nonexistence is the default assumption. Yet disbelief in all of these things is based on evidence, since we have looked in the places people used to believe they lived and/or domains they were believed to control, andand they weren't there.
The universe either came from somewhere or has always existed... If I define God as a "the thing that created the universe" how would you prove or disprove God's existence?
...then you would have to justify that definition before expecting anyone to accept, prove, or disprove it.
FWIW I was once a believer and frequently associate with believers, so I have some experience in the matter and have given it considerable thought.
The point is I'm not talking about a specific God or assuming anything about the nature of God like many religions do. Once you remove all that baggage God is just a term that means "the thing that created the universe." So to deny the existence of God you must deny that the universe was created by something. I don't see how you can make such a claim without any evidence. Last I checked there was no scientific explanation for the creation of the universe.
It's an unnecessary complication; if calling the Big Bang or whatever might have preceded it "god" provides useful explanatory and predictive power, then such a definition might gain some traction. But as it is, the universe appears indistinguishable from a naturalistic physical process.
You can believe in a god (be a theist) and still think it's not provable that there is one (be an agnostic).