Many of the city-states of ancient Greece claimed Hercules as a "founder. The most notable being the Spartans.
If Hercules did not exist, does that diminish the importance of Greek society in shaping history?
No, it simply expresses the ideals of the ancient Greeks. Whether Hercules was flesh and blood is irrelevant. Hercules exemplified ideals that resonated with the ancient Greeks and so did the stories about him.
Does the fact that the Canaanite pantheon is mythology diminishes the fact that we are all able to communicate in Latin alphabet which is a descendant of the Canaanite alphabet via Greek?
Does the fact that Osiris and Horus are mythological diminishes the pyramids and other architectural marvels the Egyptians left behind?
Does the fact that we have no archaeological evidence for Abraham or Moses diminishes the fact that the Jews gave the Bible to the world?
Does the fact that Sigurd and Brynhild of the Norse Sagas cannot be proven to be historical diminishes the maritime heritage the Vikings left behind them?
Enough said.
All of the civilizations above have both mythological, semi mythological, and historical protagonists in their cultural history. It's vain and arbitrary to pick on the mythological elements as if they cast a shadow on an entire culture and it's history. Especially since the majority of the mythological literature embodies ideals, wisdom, inspiration, and values.