In mā in li-l-ḥāʾinīna dimāʾ, mā negates, while in is understood as reinforcing the negation rather than functioning as inna, the particle that governs nominal sentences.
The prepositional phrase li-l-ḥāʾinīn is placed before dimāʾ, the delayed noun. The structure makes the declaration forceful. Grammatical emphasis, however, does not itself establish the historical or legal truth of the speaker’s claim.