كَلَّا سَوْفَ تَعْلَمُونَ
No indeed! You will come to know. (التكاثر: 3)
In this location, kallā functions as a sharp rebuke or deterrent, repudiating the preceding heedlessness. Sawfa taʿlamūn directs the audience from its present preoccupation to the consequences that will become known. The overall force is warning rather than a neutral promise of information.
Kallā can receive different syntactic and discourse descriptions in different Qur’anic contexts. Here its turn from distraction to warning is well supported; translating it as a bare “no” misses much of that force.