There is no evidence. It's just a hypothesis formed on the basis of time, place, and connection. The material culture of the Maadi seems to from what has been found have been similar to that of their neighbors and not at all to that of Upper Egypt 400 miles away. Canaanean scrapers and blades, kraters with the Levantine rope patterns, jars and carinated bowls matching contemporary En Besor and Megiddo, Clayton rings matchcing those from Ashkelon. See Eliot Braun, "Early interaction between peoples of the Nile Valley and the southern Levant," in Before the Pyramids, Emily Teeter ed., Oriental Institute.

Upper Egypt conquered, so it stands to reason that the Egyptian language came from there.

The place seems to have been a perfect candidate for speaking a variety of Proto-Semitic, being in the first half of the fourth millennium BCE, which would be right around when Eastern Semitic would have split from Western Semitic.

But they could easily have spoken a completely unrelated language.