Smothered Mate (闷杀) is a situation in which a player’s King is in check and all the spots to which the King would want to run for safety are occupied by his own pieces. Many xiangqi books use the term loosely. Allow me to first illustrate what is and what is not Smothered Mate with the two diagrams below:

It is a checkmate even if the Red King is not dominating the central file. The Black Cannon is an obstacle to the King. Jokingly put, it works for the enemy in the Intercepting Forces. This is Smothered Mate.

Here the Red King pins the Adviser, making A5-6 an illegal move and thus checkmate takes place. Otherwise, Black could move the Adviser down to parry the check. If you substitute the central Adviser with a Black Horse, it is still a Royal Checkmate, not a Smothered Mate. Without the fatal pin in the centre, Black could give up the Horse with H5-7 and allow his King to move out of check.