إِنَّهُ صَادِقٌ.
Meaning: Indeed, he is truthful.
Parsing: inna is an accusative-governing particle; -hu is its noun in the accusative position; ṣādiqun is the nominative predicate.
إِنِّي مُسْلِمٌ.
-ī is the noun of inna in the accusative position; muslimun is its nominative predicate.
An accusative-position pronoun is not necessarily a direct object.