والأحسن أن يقال: إنه نظير لتنزيل وجود الشيء منزلة عدمه
It is better to explain it by analogy with treating something existing as though it were absent.
Some people do in fact harbor doubts, yet evidence capable of removing them is present. The doubts are therefore rhetorically treated as null, allowing the sweeping denial «لَا رَيْبَ فِيهِ». The psychological existence of uncertainty and its legitimate standing are different considerations.
This also clarifies the parallel case of deniers treated as non-deniers: evidence that ought to end resistance can justify disregarding the resistance in the chosen expression.