This person may not have realized an ESTA needed in advance. He could be doing the thing people often do where they screw something up, and then, irresponsibly give a vague explanation, that they can deny later, but that leaves open to interpretation the possibility that they were somehow unfairly treated, when it might have just been their screw up.
"Denied because of ESTA" --> could actually be "Denied because I didn't get my ESTA ahead of time, and follow the clearly stated requirements. Damn, I'm stupid. Everyone make sure you read the rules before you fly! My bad."