Deontay Wilder just WENT OFF on Anthony Joshua -- saying he fought Andy Ruiz like a COWARD ... and says AJ will get "demolished" if they ever get in the ring together.
"He didn't want no engagement," Wilder said about the Ruiz fight ... "He was just scared."
"You got a 6-inch height advantage, you got an 8-inch reach advantage. Why is you running from the smaller guy?"
Wilder says Joshua's style was boring as hell -- "Punch, hold and grab like you scared of confrontation!