It was specifically a reference to
The Prince of Egypt, a December 18, 1998 DreamWorks Animation film. By the standards of book 30, this is a very recent pop culture reference for Marco.
The Reunion was published in June 1999, but by my own calculations at
http://en.wikipedia.org/wiki/Chronological_list_of_Animorphs_books, it takes place in 1998. My theory is that Marco knew of the upcoming film by way of publicity and advertising, but since it hadn't come out yet, he hadn't seen it. Knowing about it was enough to be able to make a reference to it. He said "Not to get all
Prince of Egypt on you, but... Behold!" (when "revealing" the Hork-Bajir valley.)
In addition to that, Visser One also discovered that the rest of them were humans when they have her hostage towards the end of
Visser. Marco calls Cassie (Polar Bear) by name, and Jake (Tiger) speaks in a non-Andalite way.