I'm not a REAL gymnast (I know a few). I've been a wannabe at times. Like many sports, gymnastics has changed with time. It is very different now than it was say 20 years ago.
I found an interesting paragraph on the web:

Trampoline and Tumbling History
Trampoline and tumbling can be traced to archaeological drawings in ancient China, Egypt and Persia. Over the years, a number of methods have been devised to allow man to gain time in the air and perform a variety of skills. The trampoline is one of these methods. Trampoline was not actually a competitive event until after its invention by an American, George Nissen, as a portable unit in 1936. From 1947 through 1964, trampolining was included as an event in gymnastics competitions by both the AAU and NCAA.

Gymnastics is very political (like most sports). The trampoline is not part of the popular gymnastics competition now, but it use to be. Maybe it just wasn't as popular as the other 4 events, but it seems like the gymnasts of that time could compete on any apparatus.

Don't take it too personal, we just want to make sure Alexandra gets her credit (even though I don't even know her).

Nate