Is an Orthodox Stance Jewish?

No, an Orthodox stance in boxing has nothing to do with Jews or Judaism.

Wikipedia sums up:

An Orthodox stance is a way of positioning both the feet and hands in combat sports such as boxing, karate, kick boxing, and mixed martial arts. A traditional orthodox stance is one in which the boxer places his or her left foot farther in front of the right foot, thus having his or her weaker side closer to the opponent. As it favors the stronger, dominant side — often the right side, see laterality — the orthodox stance is the most common stance in boxing. It is mostly used by right-handed boxers. Many boxing champions, such as Marco Antonio Barrera, Rocky Marciano, Floyd Mayweather Jr, Muhammad Ali, Mike Tyson, Lennox Lewis, Wladimir Klitschko, Joe Frazier and Sugar Ray Robinson used an orthodox stance.

Herschel Walker, former football player and now mixed martial arts competitor, uses an Orthodox stance (and he is not Jewish).