إِنَّمَا الْفَاهِمُ عَلِيٌّ
The one who understands is Ali alone (in the intended comparison).
Innamā is a recognized restricting device. Here understanding is assigned to Ali in contrast to other persons under discussion.
To decide the direction and scope of restriction with “innamā,” consider the complete predication—what is the subject, what is being predicated, and what competition the listener has in mind. It should not be reduced to a mere intensifier meaning “indeed” with no exclusivity.