It does not presume any such thing. It presumes simply that we had no evidence of what we "knew" to be in the bushes, but as a defense mechanism we were hardwired to believe it was there anyway. Nothing in that sentence presumes the actual existence or non-existence of predators in bushes, it only presumes that we are hardwired to first presume that something is there.this argument presumes there has never been anything in the bushes.....
This would often save our miserable short existences in the long-ago time of Grunt-speak and marriages involving clubs and hair dragging, as sometimes there really was something in the bush.
We are hardwired to presume the existence of things of which we have no evidence. It was a successful trait that, through natural selection, was perpetuated throughout the species. A portion of your brain is dedicated to this too.