Why did Allah not say: "And they said" instead of "And those who disbelieved said" in verse 26 of Surah Fussilat, and why is this expression repeated in other verses?

The rule in Classical Arabic is that the verb must always be singular, even if its doer (subject) is dual or plural. It does not attach a dual or plural marker to indicate the duality or plurality of the doer, such as "the two men stood up" (قام الرجلان) and "the men stood up" (قام الرجال). As for the dialect of "the fleas ate me" (أكلوني البراغيث) (the dialect of Tayyi'), it attaches a dual marker to the verb for a dual doer and a plural marker for a plural doer. This is a non-classical and anomalous dialect, deviating from the original rule.