The internet is full of lies. Lies like fake origins for words.
Although the origins of the word "slang" are unknown and debated, reputable etymology sources trace it back to a term for the special language of thieves (sometimes called Thieves' Cant or Peddler's French).
Of course, that didn't stop it becoming a Sudden Realisation Ralp meme. Or for former Google executing Vic Gondotra from spreading it on social media or to be accepted as fact by some.