Tafsir of An-Najm 53:51

(And Thamud) is a conjunction linked to (‘Ad). It is not permissible for it to be the object of (spared) in His saying: (so He spared not), because the negative particle ma (what/none) possesses the priority of position in the sentence, and the fa (so), as has been said, acts as an obstruction; therefore, the object of what follows it cannot precede it. It has been said: it is the object of an implied [verb] "He destroyed," but there is no need for that.

‘Asim and Hamza recited "Thamud" without tanwin (nunation), and they pause upon it without an alif. The others recited it with tanwin and pause upon it with an alif.

The apparent meaning is that the predicate of (spared) refers to both ‘Ad and Thamud collectively; that is: "So He spared nothing upon them," meaning He took them to account for their sins. It has been said: "He spared none of them," and the intended meaning is that He spared none of their disbelievers.