Calling atheism merely a lack of belief does not escape the burden of proof once someone makes claims about what is real.

A common move is to say atheism is not a claim but simply the absence of belief in God, so the atheist has nothing to defend and the believer carries the whole burden. As a description of a mental state that is fine, but it does no work in a debate. The moment someone says the universe is uncaused, or that mind comes from matter, or that there is no purpose, they are making claims that need support. And does God exist is a question whose denial is also a position. Whoever asserts something, for or against, owes reasons. Hiding behind lack of belief avoids the actual question.