Cruz did “congratulate” Trump for winning the nomination, at the beginning of his speech. In the absence of any words of support or endorsement, it was just a perfunctory as what a candidate may say to the election winner from another party.
I shouldn't be so nit picky, but the man is a lawyer who makes his living off of other people's money with his words.
If he actually wanted to congratulate, he most certainly knows how to do so in such a way that all would have heard it.
However, his speech seemed to indicate that if he really wanted to he would have, but he didn't, so he won't.
Otoh, maybe he'll get around to it later maybe, just like with that endorsement pledge. He'll do that later, too.
He said that he wanted to and his word is his bond.