When the author says the three stage rocket is simple, he is saying that that rocket is simple, not that three stages is simple. The author's English is correc

When the author says the three stage rocket is simple, he is saying that that rocket is simple, not that three stages is simple. The author's English is correct with the meaning but can be misread. He hasn't used English that can be unambiguously parsed, presumably because he is not a native English speaker - I assume he is a native Persian speaker. The author is obviously saying that the rocket currently being deployed in two stages is more advanced than the rocket currently being deployed in three, which is why the former will be capable of more when it too is deployed in three stages. You just chose to parse it the wrong way, and then call the author's writing just plain wrong.