You're mixing things up, I think. It's fine to have a reveal somewhere in the story (or else we'd never have the thriller genre!) but you need to be both consistent and timely in doing so. The point you bring up is to do with the first part of that - if your character does un-cyborg-like things or something that would have revealed him as a cyborg earlier, your reader is going to feel cheated because you've deliberately evaded revealing him in situations where it would happen, or made him act out of character.
The timely part is about how you handle the reveal - it has to be handled well or else it has no dramatic impact, and in the worst cases, the reader feels cheated because you've been either holding back, or it's been blurted out too early and everything that follows is simply pedestrian.
Imagine, if you will, a cliffhanger at the end of an adventure serial: the hero is in a pinch! Oh no! The acid is burning through the rope holding a twenty ton weight above him! He gets one hand free of his bindings! The acid is almost through! He tears furiously at the other hand! The rope breaks! The weight falls! We cut to black!!!
The audience show up the next week, clutching their popcorn, desperate to find out what happened to our hero! They see...a two hour retrospective on the history of flower-pressing.
The News that night reads "Angry Mob Burns Cinema To Ground"
The point is that, eventually, you *are* telling your readers everything. A cautious and consistent control over how you tell them is, in essence, storytelling.
