^If, if, if. The meaningful question is, is it likely? What are the odds that some charismatic megalomaniac looking to start trouble would pick that particular issue to stir people up over? I mean, charismatic tyrants rarely rely on anything so complicated to win over the masses.
"Humanity is our enemy because they created the Borg!"
"Uhh, excuse me? The Borg existed thousands of years before humans went into space."
"But the humans went back in time!"
"Why would they do that?"
"Look, it was an accident! There was a Caeliar time vortex that went wrong."
"Who are the Caeliar?"
"They were a superpowerful race that joined with the humans to become the Borg."
"So isn't it the Caeliar we should hate?"
"No, dude, the humans are totally to blame because they sabotaged a Caeliar city and triggered the time vortex."
"Why did they do that?"
"Because the Caeliar were holding them prisoner!"
"So... again, doesn't that make it the Caeliar's fault?"
"The Caeliar aren't here anymore!"
It's too feeble. Too complicated to rationalize. Charismatic warmongers rely on Big Lie propaganda, and the first principle of that is to keep your rhetoric simple, to appeal to the lowest common denominator. You don't want some complex chain of causality that you need to explain in order to justify the conclusion that humanity is the enemy -- you want something simple and obvious, something the listener can pick up on right away and relate to viscerally. If you want them to hate humanity, you need to give them a more immediate, less complicated reason. Say, they're trying to take over the galaxy, or they spread disease, or whatever. Blaming humans for the Borg just doesn't cut it as a propaganda tool, because it's not straightforward enough. It's also hard to justify when the immediately evident fact is that humans played a key role in eradicating the Borg once and for all. That's the big headline, the sound bite that the listener can easily grasp. And that's going to be more effective for propaganda purposes than some convoluted history lesson that can, at best, only feebly implicate humanity in the Borg's distant origins.
Obviously you don't pay attention to the nightly news, where a sound bite is all you get. You also conviently do not factor in op-ed pieces and spin doctors or the fact the the millions that are now disenfanchised may not have access to all the data.
Here's another soundbite that is easy to grasp, "Humanity creates Borg". Not necessarily true, not necessarily untrue. But it is something someone could run with. The hungry masses in Germany after World War I didn't seem to remember that they started it and bought into Hitler's propaganda pretty quickly that it was the Jews fault. All you need is that 'boogey-man' to get the ball rolling. No matter how illogical it may seem.
And how much information do you allow to get out there? The contact in 2063? The contact in 2154? The Hansens chasing the Borg 15 years before anyone else knew about it? The fact that a starship captain let two chances of possibly taking down the Borg go by. Plenty of juicy stuff to work off of.