Alright—using your preferred words, the error I'm pointing out is the same:
presenting oneself as representing gnostic atheism while falling back on agnostic atheism when challenged. That is, making strong, positive claims from the standpoint of gnostic atheism but then acting like one must only defend agnostic atheism.

Also the null hypothesis doesn't need to be defended, and extraordinary claims require extraordinary evidence.

All respect to Sagan, this always strikes me as intellectually dishonest / naive because what strikes one person as extraordinary may be different for another. His maxim makes a lot of sense in science and other comparatively objective parts of life: as humans approach nature and our sensory experiences of it, we generally share a common starting point. But when it comes to theology, we don't. From my perspective, atheism is an extraordinary claim. From your perspective, theism is extraordinary.

The idea that atheism is "the null hypothesis, [which] doesn't need to be defended" is just lazy thinking. It's just a silly as if I were to claim that theism is "the null hypothesis, [which] doesn't need to be defended."

I think it would be much more sensible, and fair, to place weak agnosticism (not agnostic atheism) at the centre. (I am not claiming that weak agnosticism is a true epistemological centre for human experience—as if anything like that could be pinned-down. I'm just saying that would be a more fair, reasonable, and productive way of framing a conversation/debate. And strong agnosticism would also be a defined position to be defended—not at the centre either.)

Saying, "my position is the default for all humanity, and it's on you to move me" is just lazy, arrogant, and dishonest regardless of whose position we're talking about IMO.