The official response clearly blames the coronavirus for the death, accooding to the Thai Public Health Ministry. Here is an excerpt from the Bangkok Post article I posted above. As you will read, COVID-19 is certainly considered the cause of this patient's death (bolding is mine for emphasis):
Originally Posted by Barnet
"The patient had been hospitalised since Feb 16 at a private hospital and transferred to Bamrasnardura Infectious Diseases Institute and died on Saturday after multiple organ failure.
The Public Health Ministry is still looking into the role played by the coronavirus in the fatality, Dr Suwannachai said.
Tests for the coronavirus came back negative since Feb 16 but “the damage was already done to his body” from the earlier bout of infection by the new disease, said Tawee Chotpitayasunondh, the department adviser."