In the Japanese manual, Sypha's gender was also left ambiguous, if not implied to be masculine. And if you look at the Japanese artwork, she doesn't look particularly male or female, either.
I doubt whoever wrote the American manual had any homage to Metroid in mind. Knowing how out of it Konami's manual writers were back then, he probably didn't even play the game, just took the Japanese manual at face value, and unknowingly propagated the misleading profile.