The union leader is the direct representative of that small group of people. They have authority directly from the people in order determine what the people want them to do to represent their business. Trump is their representative, too, however he is farther removed politically from that specific group of people.
The union leader knows the truth about the deal. He knows what the deal cost the state, and how many workers are still losing jobs. The union leader is being intelligent and making sure that the people know that Trump should not be hailed a hero for this deal that really wasn't so great after all.
Yes, the union leader representing workers better than Trump in this case. This is not surprising, as the union leader in this question has dedicated his professional career to representing the interest of his workers, while Trump has been found to have refused rightful payment to workers who have done work for him.
The Union Leader representing Carrier workers did not need to call Mr. Trump a liar. Carrier jobs were saved. Not all of the jobs, but some of them. Accordingly, it would have been better for the Union Leader representing Carrier workers to recognize the success in saving some of the jobs and avoid the hyperbole directed at Mr. Trump.