Allegedly 40% of American families could not handle a $400 emergency cost. So the "booming" economy is a bust for them. True?

 Economy has been a bust for the bottom end for many years. 
Wages are starting to come up now for the bottom.
Not being able to handle an emergency comes down to poor wages (working on that) and poor spending/savings habits.

I'd say it has more to do with individual spending (savings) habits, than wages.
Years ago, I worked at a job that paid $20/day + room and board.
Some of my co-workers getting the same pay, lived paycheck to paycheck because of their spending habits.
I was able to save enough to buy a new car, due to my savings habit.

Their spending priority was to "live in the moment" ... mine was to "plan for the future".

I'd also say it should be a matter of educating people before they get their first job.

Not sure how anyone would came up with such a percentage.  Especially when the only thing I know is they never interviewed or surveyed me.  So I don't trust such figures.  But many people would not be capable of or in the position to enjoy or take advantage of a booming economy.  Or any kind of economy.  $400 does not seem like much to spend on an emergency.  A cheap emergency.  But if people have not put any money by then of course they will not have it.  Perhaps they live beyond their means or they just don't make enough to begin with.  To describe anything in such general terms will of course not apply to absolutely everyone.  Just like sometimes some of us are doing well even when the economy is said not to be booming. 

Well that $400 could be gone in a second if the person hits a pothole on a road that is not being fixed because Trump is being a big baby on infrastructure. Cheers!