At the time, it was known that water consists of 2 atoms of hydrogen and 1 atom of oxygen giving the molecular formula H₂O. It was also known that the combustion of hydrogen in oxygen yielded 2 volumes of water. If the atoms were not diatomic, it would make sense that 2 volumes of hydrogen and 1 volume of oxygen would result in 1 volume of water (not 2).

Does that make sense? Let me know…