Sheng Long is a really lazy miss translation. It was supposed to say you must defeat Shoryuken to stand a chance.
The original Japanese win quote for Ryu was "IF YOU CANNOT OVERCOME THE RISING DRAGON PUNCH, YOU CANNOT WIN!" (昇龍拳を破らぬ限り、おまえに勝ち目はない!)
The words sho ryu from (昇龍 rising dragon) from Shōryūken (昇龍拳) were translated to "shēng lóng" in Chinese. This was translated to "YOU MUST DEFEAT SHENG LONG TO STAND A CHANCE." in the English arcade version of Street Fighter 2. So (at least in the English version) it sounded like "SHENG LONG" referred to a person instead of the shoryuken attack. So that's why there's the confusion, because it's translated between three different languages. The English SNES version changed the quote to "YOU MUST DEFEAT MY DRAGON PUNCH TO STAND A CHANCE." (this was because the SNES version was released after the EGM prank was published). But, despite this, the English manual for the SNES SF2 referred to Sheng Long as being Ryu and Ken's teacher.
The closest thing to a Sheng Long appearance in a Street Fighter game is Gouken in Street Fighter 4.