I didn't notice this incident (I watched the game) but I take it the ball was roughly knee high. If not ignore this post.
Imagine a ball is coming down to a defender with no attacker near him, he makes it bounce head high and then heads it back to the keeper. A much easier and natural play would have been to control it with the foot on the bounce or second bounce. No one even thinks of trick to circumvent the laws here. If a ball is already knee high or thereabouts, kneeing it back to the keeper shouldn't even raise thoughts of trick to circumvent the laws. The reason 'deliberately' kneeing it back to the keeper is allowed is to cover this situations. Otherwise what other reason would it be allowed for? The general times players knee a ball is when it is knee high.
To be clear the word trick is not for referring to the act of playing the ball but for the act of circumventing the law. For example after a direct cross from an attacker if the defender heads the ball thrice before heading it again to the keeper, this act is not a trick to circumvent the law even though heading it four time is trickery.