Hey @JamesHunt, great question! I want to make sure that I understand your question first. The way I understand it is as follows:
The Christian says, "see, look, science shows us in the Big Bang that the universe came into existence out of nothing, ex nihilo. To which the non-Christian replies, “Ah, but you misunderstand the Big Bang. The Big Bang shows us the beginning of our universe; there was a time at which our universe exploded into existence. However, prior to the existence of our universe, there existed a super-condensed “ball” of matter and energy, out of which our universe came to be. And so, that simply shows us that we do not know where that ball of super-condensed matter plus energy came from. You, Christian, have no leg to stand on in saying that the universe was produced ex nihilo.”
Is that the argument that you are asking about? And is that a fair way to state it?
The way I see it, I think there is a confusion of an argument on the basis of pure reason (used by the Christian, in this case) and an argument on the basis of scientific observation (used by the non-theist). The rough sketch of the argument based on reason is that for every material thing we see, there is a material cause of it, i.e. the grenade came from the grenade maker; the grenade maker came from their mother and father, etc. The problem is that this leads to an infinite regression of material effects, which seems to be a logical impossibility. Therefore, the Christian contends that pure reason seems to confirm that at some point there was nothing, and then there was something; that is to say, creation ex nihilo. With the observation of the Big Bang, it seems reasonable enough to point to this as the moment of creation of the cosmos, prior to which there was nothing.
The other argument, the argument on the basis fo scientific observation, goes something like this. With the Big Bang, we see the moment at which the universe came to be; however, all of our measurements and calculations point to the fact that prior to the Big Bang, all of the matter and energy of the universe already existed, just in a hyper-condensed ball of matter and energy, the so-called singularity where all of our laws of physics did not apply. Therefore, the contention is that the Big Bang doesn’t show us the moment at which something came from nothing; it simply shows us the start of our universe, but the matter and energy already existed prior to this.
The confusion of the arguments, as I started with, is that the scientific argument misunderstands the nature of the logical argument. The logical argument says “look, the Big Bang seems to be a good point at which we see the beginning of something out of nothing” whereas the scientific argument contends, “No, matter and energy already existed before the Big Bang, so you are wrong.” But it fundamentally mis-understands that the argument is based on the logic that at some point a material effect must have had an immaterial cause. If the Big Bang wasn’t that first material thing, perhaps the singularity was that first material thing. Perhaps at some point we will be able to point beyond the singularity to material stuff that existed priorly. Nevertheless, none of these while negate the logic of the argument - certainly at some point, there had to be an immaterial cause of the material. Simply because we haven’t or can’t observe the actual beginning of the material cosmos doesn’t mean that ex nihilo creation is an impossibility or at odds with scientific work.
Hopefully that very brief sketch of the arguments will help bring some clarity to your question - does it make sense?