What is the semantic difference between "إن الكافرون إلا في غرور" (Indeed, the disbelievers are but in delusion) and "ما الكافرون إلا في غرور" (The disbelievers are naught but in delusion) in Qur'anic usage?

There is no difference in meaning between the negating particles "in" (إنْ) and "ma" (ما). Exegetes interpret "in" here to mean "ma." Al-Suyuti and the commentators on the Alfiyyah mentioned that the negating "in" is interpreted as "ma." Ibn Ashour uses the verse as evidence that the disbelievers are in perpetual delusion, and this is a restriction intended to reverse their belief in the protection of their deities. He also pointed out that negation with "in" is less frequently used than negation with "ma."