It's not completely ludicrous: there needs to be some speculation about what is and isn't possible given it's a sci-fi universe, but the human brain can do orders of magnitude more compute for the energy it uses compared to silicon, and energy is the one thing the machines are short on after the humans blocked out the sky. (Ok, you need to handwave power sources like nuclear being unavailable). The main issue is how do you use neurons for this, why do the humans need to be conscious for this, and how much of their brains can the machines actually usefully use, considering the previous point?