Maby FRED SEXP's and LISP sexp's aren't the same, so they are not pronounced the same.
REagardles, official documentation sez sex pee
First of all, FRED sexps and LISP sexps
are the same. In fact, practically all of the non-game-specific sexps come directly from LISP.
Second of all, just because something is official doesn't mean it's correct. The IMDB example should have clued you in to that fact. If you need a second example, the Official FS1 Strategy Guide says that the SD Tantalus is a Cain-class cruiser.
Third of all, the documentation was written by Peter Drake, who is a writer, not a coder. Thus it is quite probably that he may have been ignorant of the word's origin.