That they are the companions of the Fire is in the nominative case as an apposition to the saying 'the word of your Lord.' It means that just as that obligation was necessary for the disbelievers to be among the companions of the Fire. Its meaning is: just as it was necessary to destroy them in this world by the complete punishment, so it is necessary to destroy them by the punishment of the Fire in the Hereafter. Or it is in the accusative case by omitting the لام of purpose and connecting the verb. And those who disbelieved refers to Quraysh, and its meaning is: just as it was necessary to destroy those nations, so it is necessary to destroy these, because there is one reason that unites them, which is that they are companions of the Fire. It has been read as 'words.'