The statement may appear to be a casual guess on how the world began but it actually contains deep profoundity. Countless figures had been searching for the way the universe began and through this seemingly infinite search, they found myriad of different clues. The old man was not impervious of these clues. He himself dug deep about this topic as well.
The statement from Z connected too many dots in his mind that he is currently beating his mind for not thinking of this origin. Of course, there is no way for him to conclusively determine whether this hypothesis is true or not, but the only thing that he needs is a logical explanation. This is enough for him to proceed on researching this area. It is much easier to search when the area is specified.