Wasn't their excuse respiratory failure? He died from respiratory failure.
Later came to light that the Derg executed him. Bunch of soldiers took turns strangling him and buried him in the palace, apparently in the toilet. The assassination order was at least confirmed in leaked DERG documents but who knows how true the strangling part is.
In 2006, 67 former members of the Derg said that Selassie was strangled on August 26, 1975. link