In his book التبيان في الناسخ والمنسوخ في القرآن المجيد, Aş-Şa`di Al-Yamaani points out on page 71 that the Arabs often use the negative article
لا, not for negation but for negativity. As an example, he quotes a poem by Qays ibn `Aasim Al-Minqari in which he says,
بيوم جدود لا فضحتم أباكم --- وسالمتموا والخيل تدمي شكيمها
Translation: On the day of Judood (a battle named after a place belonging to the Banu Tameem tribe), you
did not disgrace your father, and yielded [to your enemies] when the horses were staining their bits with blood.
The meaning actually is: You disgraced your father by your quick surrender; shame on you.
He says the Quran showed the same style, e.g.,
where the words say "So that the People of the Book may
not know..." but the meaning is "So that the people of the Book
may know ... They ought to know."
He mentions this to explain the opinion of As-Suddi and Yahya ibn Al-Husayn aka Al-Haadi who said the meaning of
يطيقونه (can barely do it) is لا يطيقونه (cannot do it).
That is an interesting point, and it's good to know about this Arabic style. But it amounts to the same thing in the end: The word refers to a borderline action, i.e., they can hardly do it.