It all seems very trivial but West should get into the habit of playing the correct card on the 3rd round of the suit.
West holds K10 after both the Q and J have been played. West’s cards, therefore, are of equivalent value.
By playing the King under the Ace, South doesn’t know who holds the 10.
By not playing the King under the Ace, South certainly knows who holds the King (because East has failed to play the King on the first two rounds of the suit)
Holding cards of equivalent value, you should play the card you are known to hold (the King).
Defenders who disguise the position of cards make the play of a hand more difficult for declarer. Good defenders will do this automatically..