It seems to me that the statement quoted is essentially true. Macbeth is a man of conscience but with a fatal weakness of ambition. However, his ambition seems to be of a peculiar kind. He seems more ambitious to please his wife and to satisfy her ambition than he is to satisfy any particular ambition of his own. His wife is even more ambitious than he is, but her ambition seems to be of a special nature too. She wants social preeminence, but she can only get it through her husband. Together the two characters seem more ambitious about genetic success: they want to start a new royal dynasty. Evidently they don't have any children but expect to have some in the future. Banquo becomes their enemy because of the witches' prophecy that his heirs would form a succession of kings. This is a familiar situation in which two people who would be relatively harmless individually form a lethal combination when joined together. Macbeth would not have committed the murder without the relentless encouragement of his wife, and she could not have achieved her ambition without her husband. Lady Macbeth seems stronger than her husband in almost every way.