يَعْلَمُونَ ظَاهِرًا مِنَ الْحَيَاةِ الدُّنْيَا ... وَلَكِنَّ أَكْثَرَ النَّاسِ لَا يَعْلَمُونَ
They know the outward aspect of worldly life ... yet most people do not know.
The forms yaʿlamūn and lā yaʿlamūn present affirmation and negation of knowing. The two assertions are not contradictory because the respective objects of knowledge are different: outward worldly affairs versus what the wider context says they neglect.
The rhetorical contrast exposes the limitation of a particular kind of knowledge, rather than merely placing a positive word next to its negative.