When thinking about the unification of Egypt, Watrall was talking about how it most likely occurred over a period of time, and not in one swoop. But, discoveries like the Narmar palette seem to suggest that the unification was quick and took place under one leader, Narmar. Within the Narmar palette, it depicts a leader wearing the bulbed white crown of Upper Egypt, then later he is depicted wearing the white crown combined with the red crown of lower Egypt. Some historians may look at the Narmar palette as conclusive evidence that the unification of Upper and Lower Egypt was achieved under one ruler.
Though it is more likely that the unification of Upper and Lower Egypt took place over several years, and with proto-pharaohs ruling before the government was established. The Narmar palette, Scorpion mace heads, and the Narmar mace heads depict the political unification of Upper and Lower Egypt, though it is unclear who began the unification process. Within the Scorpion mace heads, a pharaoh is depicted wearing the white crown of Upper Egypt, then in the next mace head the same king is depicted wearing the red crown of Lower Egypt. Similarly the Narmar mace head depicts similar evidence.
Yet despite all of the evidence pointing to the unification of Upper and Lower Egypt under Narmar and the Scorpion pharaoh, the mechanics of the unification remain unclear. Some scholars debate whether Narmar and the Scorpion pharaoh are the same person, or different people ruling at the same time. Other scholars argue that they are separate rulers. Another interesting debate is whether Narmar is a proto-pharaoh from Dynasty 0, or the first Dynasic pharaoh.
I imagine similar debates will occur when people look back to the early days of any civilization. Even the early history of the United States is held in contention, so it makes sense that such questions are such difficult to answer.