What is odd is that they have perhaps most benefited from ambition—if not always their own then that

题目

What is odd is that they have perhaps most benefited from ambition—if not always their own then that of their parents and grandparents.
哪位强人能帮我分析一下这里破折号之后If引导的从句啊?好难啊,根本找不到语法点
这句话是考研英语2000年TEXT5第一段的一句话，翻译出来是：“奇怪的是也许已经从雄心壮志中获益最多的正是他们——如果不是他们自己的雄心，那么就是他们父母的和祖父母的。”if not always their own then that of their parents and grandparents这句是固定搭配吗？用普通语法讲不通啊！

答案

if (the ambition) is not always their own then (the ambition is) that of their parents and grandparents
省略了一些主语以免重复,所以会感觉缺了成分