Thunderbolts: Religion tells you what to believe, ... not the truth!

Exactly what it does mean is: Religion uses truth disingenuously, chopped and cropped so not to appear insane... But the whole theme has enough distortion to alter the perception of the real truth to compel the people towards the acceptation of their objectives of control without rebellion.
Let's take the flood, it didn't produce a great killing in Kish nor in most of the other cities of the Sumerian land. Except at the very south of it, the people of Eridu were born the brunt of the flooding which a that time was more of a gentle rise which allowed them to seek higher ground towards the west. The death of the populations at Kish were consequence of two things, starvation (which decimated the population in a long period of years, more than two score) and pestilence, as the river wasn't running anymore by them, but the effluents of their human activity still went into it, which in turn it was carried by the sporadic rains down the river to the other cities. The record talks of fifty years of the curse.)
What the Noah story talks about it?
What true is in the Noah story? Nothing worthy to mention.
Now, going to the flood of Gilgamesh, it is an ancestral theme imported from India and beyond which refers to a tsunami like flood of the land... following an earthquake and a catastrophic volcanic event in the Indonesian Islands. Read it carefully and you will find the exact description of such event confirmed by the current research. Again, nothing to do with religion. Unless you want to take advantage of the credulity of the people and shamelessly use it to that effect.