You assume every big league player is a study of the game who knows everything there is to know about mechanics. I highly doubt that's the case as many players get there based on god given talent (which is why we have huge coaching staffs these days). Finally, Pedro may understand his own mechanics - I fully expect he did.....but every pitcher has different mechanics. Castro is like 7 inches taller than him....does he know and understand the mechanics for all the different types of pitchers in baseball?
Reality is - he's likely see a couple of videos on the kid, sees a tall, projectable body, a 97 MPH fastball with movement and a good change up and says "that kid looks special" - just like you and I do. Good news for Pedro is in 2 years if Castro flops - nobody will remember or call him out for being completely wrong about him.