عَادَ الطُّلَّابُ لَا يَدَّعُونَ الْإِحَاطَةَ بِكُلِّ شَيْءٍ.
The students returned without claiming to have observed everything.
The negative imperfect lā yaddaʿūna describes their state at the time of return. Negative imperfect circumstantial expressions allow analyses and linking patterns that should not simply be assimilated to the rule excluding a direct circumstantial wāw before an affirmative imperfect.
The key issue is whether the clause qualifies the main event or states a second independent predication. That choice governs the syntactic and rhetorical interpretation.