I agree that the 2nd test shouldn't be necessary in this situation, but to be fair, the traveller could have gotten infected during the trip to the airport (there was a news story couple months ago on here about how someone died of covid and in one of their tweets they thought the cab driver that drove him to the airport infected him because the cab driver was coughing badly and looked sick overall)
Also, there's a possibility that the test papers may be faked (either by individuals wanting to save money, or are infected and didn't want to cancel reservations , flights etc or just wanted to be in thailand no matter what)