Tafsir of Ad-Dukhan 44:50

(Indeed, this) meaning: this punishment or this matter that you are currently in, (is what you used to doubt) meaning: you used to harbor doubts and dispute regarding it. This marks the beginning of speech from Him—the Almighty and Majestic—or it is part of the spoken utterance.

The use of the plural form is in consideration of the meaning, as you have previously heard that the intended meaning is the category of the "sinner" (al-athim).