The alien transmission is an alien defensive weapon

They either know of humanity specifically and has been for probably thousands of years or discovered that all life is RNA/DNA based and the transmission will work on any race that reaches a technological state to be able to receive and decipher it.

A race that can not kill is harmless. We, under a hive-mind like shown in the series would never attack or be hostile.

Why? My idea is that any technological race would eventually reach a point were they could build weapons that can not be defended from. Just lunching a bowling ball and accelerate it to near light speed is a planet-killer and more or less impossible to defend from. No matter if they are 15 thousands years ahead of us. This is if the in-universe is hard sci-fi and FTL does not exists or other magical technologies that would break current physics hard.