I completely disagree. Gen X goes all the way back to the 1960s. Whereas millennials are commonly referred to as those having been born between the early 80s - early 2000s. The term Millennials refers to the graduation year of 2000, meaning that 1982 would be the commonly referred to start date of birth for Millennials. 1996 appears to be a popular cutoff putting Millennials at 5-20 years of age at the time of 9/11.
Length of generation is something to consider. In terms of human life, the average generation is 25.5 years. In terms of sociology, the longest generation since 1900 is 23 years with the shortest being 17 years. Using the definition you proposed, Generation X would last from about 1961 - 1995 or 34 years. I just don't see that happening. The Pew Research Center uses the dates 1981 - 1996. The men who coined the term, William Strauss and Neil Howe, place the dates as 1982 - 2004. Assuming the longest possible length using these two commonly referred dates, the generation would be 22 years in length putting Millennials between 14 years old and 37 years old. The shortest length of generation y combining these dates would put millennials between 18 years old and 36 years old.
I really don't think Millennials will be thought of as having no understanding of 9/11. In fact, many researchers make the exact opposite proposal, that Millennials were largely affected by 9/11 in terms of psychology and how they live their lives.
I've read several of their Books, they are great reads. However, I don't get the confusion. We are very close on dates. Part of the problem is that some people have been shrinking the age groups and I think they have gone too far. The original book had the definition of Gen X from 1964-1984 (20 years) and Millennials were (1984-X) and that would have put them around 17 when 9/11 happened. It's not that they don't have an understanding of the specific event but that they didn't really know how things worked before the event. A good example would be remembering a time when you could fly on airplane without ID, getting groped, and being able to smoke. Most Gen X's can remember flying this way while Millennials would not.